GUI: Fix ListWidget::getEditRect's returned height.
GUI: Fix EditTextWidget::getEditRect's returned height.
GUI: Fix character redrawing behind caret in EditTextWidgets.

This fixes an ugly y position change when the caret is moved to a character in
an edit text widget.
GUI: Fix out-of-bounds check in EditableWidget::drawCaret.

The line "y + editRect.height() + 2" is not included in drawing anymore. Thus
it is allowed to equal EditableWidget::_h.
GUI: Draw caret over the whole height of the edit rect.

This improves the look of the editable widgets.
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.
AVALANCHE: Fix savegame version checks