Throw most MainWindow functionality into MainWidget
Move the MainWidget ui into its own file
MainWindow: Remove 'using' line
make all pages invisible by default

Otherwise, they screw up the order when constructing the MainWidget, and
we do a show_all on the MainWindow anyways.
MainWidget: Use the stack's visible_child property

instead of the visible_child_name
Call switch_page directly on the MainWidget

Unfortunately, we can't just proxy it through the MainWindow.
MainWindow: Remove leftover comment
Remove some mock adjustments
MainWindow: Add change_account function
Improve account switching
New AccountCreateWidget
TweetListEntry: Delete tweet on first try...

... if the user's been using the menu item
MainWindow: Show the left separator again
AccountCreateWidget2: Show info label
MainWindow: Don't show app_menu_button if account == null

In this case, we are in "account setup mode" and we don't want to
show anything but the AccountCreateWidget.
Merge branch 'main-widget' of https://github.com/baedert/corebird into main-widget
Show added account in the SettingsDialog
SettingsDialog: Fix AccountListEntry when new account added
MainWidget: Remove old code
MainWindow: Exit early in delete handler if account == NULL