GUI: Document EditableWidget::getEditRect.
GUI: Fix undrawing caret glitch when the edit text is inversed.

This is prominently visible in the list based save/load chooser since the
edit string is drawn on a special green background there. When the caret is
at the end of the edit string this would result in the green color missing
at the place of the (undrawn) caret. To avoid this we simply draw a fake
space now.
GUI: Do not draw text outside edit rect in EditableWidget.