We don't want to trigger an actual selection change until either
confirming the new selection by keyboard or clicking on it.
Dismissing the dropdown should have no effect on the current
selection.
Fixes#4657
Instead of TextEditor knowing about the ComboBox button on the right
hand side, we now make ComboBox inherit from GUI::Frame, and make the
inner text editor widget frameless.
This allows us to place the button ourselves inside ComboBox without
any frame artifacts, and TextEditor no longer needs to keep track of
the geometry of that button.
