Introduce view models

.../kotlin/org/briarproject/briar/desktop/BriarDesktopApp.kt

src/main/kotlin/org/briarproject/briar/desktop/BriarUi.kt

...in/kotlin/org/briarproject/briar/desktop/DesktopModule.kt

src/main/kotlin/org/briarproject/briar/desktop/Main.kt

src/main/kotlin/org/briarproject/briar/desktop/UI.kt

...g/briarproject/briar/desktop/dialogs/ContactsViewModel.kt

...in/kotlin/org/briarproject/briar/desktop/dialogs/Login.kt

.../org/briarproject/briar/desktop/dialogs/LoginViewModel.kt

...in/org/briarproject/briar/desktop/dialogs/Registration.kt

...iarproject/briar/desktop/dialogs/RegistrationViewModel.kt

...arproject/briar/desktop/paul/views/BriarUIStateManager.kt

...iarproject/briar/desktop/paul/views/PrivateMessageView.kt

...lin/org/briarproject/briar/desktop/BriarDesktopTestApp.kt

...otlin/org/briarproject/briar/desktop/DesktopTestModule.kt

...org/briarproject/briar/desktop/RunWithTemporaryAccount.kt