This improves the app updater so that it doesn't rely on unreliable
system notifications. Now, a banner in the tab bar will display when an
update is available. Clicking this will prompt the user to restart the
app and complete the installation.
This also updates the tab bar to move to the smaller tab size earlier so
we don't need to make the tab bar scrollable as much.
This adds a new global atom to track whether a window is in full screen.
It also updates the behavior of the tab bar so that it will only add an
extra left indent on macOS windows that are not in full screen.
Otherwise, the indent will be much smaller.
