Beautify main window UI and also make it responsive

This combines the contacts and the chat ui into one window. As a source I used Fractal's main window UI file which is OK due to them also using the GPLv3 as license. We need to make sure to quote them in Briar GTK's about page, though.

Fractal currently looks like this:

source

added S: in progress UI/UX Usability labels

added 1 commit

• 05b4d880 - Bump libhandy to newest version

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added 5 commits

• d03ef507 - 1 commit from branch master
• c3fba003 - Add libhandy
• a2469ed8 - Switch to main window ui by Fractal - not working yet
• eedd9573 - Delete room list search bar
• 5711dd99 - Bump libhandy to newest version

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added 1 commit

• 808818c8 - Get it working

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added 10 commits

• 808818c8...7a43e916 - 5 commits from branch master
• b2625b79 - Add libhandy
• 031e4b13 - Switch to main window ui by Fractal - not working yet
• a3cd46e5 - Delete room list search bar
• 17497bb0 - Bump libhandy to newest version
• c7827d7c - Get it working

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added 6 commits

• 6763a995 - 1 commit from branch master
• d9b5662a - Add libhandy
• 9ca036c8 - Switch to main window ui by Fractal - not working yet
• 3ba8afe7 - Delete room list search bar
• 4589a2a7 - Bump libhandy to newest version
• eef662d5 - Get it working

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added 1 commit

• b57553dc - Make HeaderBar work

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added 2 commits

• 00e97c94 - Improve style
• bc76a4c2 - Continue to get it working beautifully

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added 1 commit

• 1e933975 - Continue

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added 1 commit

• cb0cd06a - Continue

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added 12 commits

• cb0cd06a...5794c96b - 2 commits from branch master
• fd822b29 - Add libhandy
• 499af814 - Switch to main window ui by Fractal - not working yet
• e9a52e68 - Delete room list search bar
• 44df8f2f - Bump libhandy to newest version
• 9e619832 - Get it working
• 26984609 - Make HeaderBar work
• 3eb9f26b - Improve style
• 9a10e4e0 - Continue to get it working beautifully
• b3174e17 - Continue
• bef5d3b6 - Continue

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added 2 commits

• d12be8e6 - Move get_contact_name to own function
• eda349e5 - Get it all working

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added 13 commits

• ffe817bd - 1 commit from branch master
• 6ff079bd - Add libhandy
• 31afb185 - Switch to main window ui by Fractal - not working yet
• d7e8417a - Delete room list search bar
• ad3228d7 - Bump libhandy to newest version
• 1533868c - Get it working
• d136ec39 - Make HeaderBar work
• fd0900d8 - Improve style
• 88f0b08d - Continue to get it working beautifully
• 9938fd08 - Continue
• 4f68ece7 - Continue
• 20a567c9 - Move get_contact_name to own function
• 0d877347 - Get it all working

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added 1 commit

• 51120f81 - Revise, but also break stuff

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added 1 commit

• e092c638 - Rename partner of main content in header bar, too

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added 3 commits

• 0436fff7 - Move WindowActions to own directory
• 269100e5 - Rename chat to private_chat and move into widgets
• 8b5fc0da - Move message creation to own class

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added 4 commits

• 627ee820 - Close chat with Ctrl+w instead of escape
• e3426e53 - Add shortcut for closing Briar with ctrl+q
• a3bf7c2a - Create own class for contact row
• c5fb9f68 - Further refactor private chat and main containers

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added 4 commits

• 3d61f99b - Make window property of application public
• 4e890a75 - Add actions for application
• 886acdb8 - Add libhandy to adapt to smaller screens
• 7cbc6a35 - Combine contact list and chat view into one window

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added 1 commit

• b05cce49 - Build libhandy in GitLab CI

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added 1 commit

• a8f175af - Build libhandy in GitLab CI

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