ButtonWidget has on-click event
Rename mouse{down,up} event parameters
Fix division by zero in BoxWidget align code
Fix non-fixed-width children of ScreenWidgets
Merge branch 'ui-semantic-events' into devel
Take border into account on BoxWidget layouting
Some utilities in Rect and Border
Hint border lines, avoids blurryness and is more correct
Send click events after MouseUp events

Doing it the other way round breaks mouse capturing and can trigger
unwanted clicks.
Offer flag which indicates whether any repaint has been done
Don't register nclicks if mouse moved too far between two clicks