Consistent naming "Briar" throughout app

The following discussion from !274 (merged) should be addressed:

• @ialokim started a discussion: (+3 comments)

  While trying out the installer generated by the CI, I noticed we use "Briar" as the application name during installation and also for the .desktop file for Linux, but the title of the main window is "Briar Desktop". Not sure if we should instead be consistent.

TL;DR: We probably want to use "Briar" everywhere instead of "Briar Desktop".