Whitespace
Facilities to retrieve label text dimensions
Correctly align text in labelled widgets
Default implementation for WidgetBase.getDimensions()

Returns the desired dimensions a widget wants to occupy.
Provide intermediate box classes

These cannot be directly addressed by CSS rules, thus they are suitable
for subclassing entirely different widgets from them.
Do not collapse padding with margin
Fix typos
LabelledWidget returns dimensions
New, cleaner Theme management

The root widget now has a theme assigned and that gets automatically
broadcast to all child widgets.
Send on mouse{enter,leave} events to hovered items
Allow Auto value for CSS width/height
Clear focus if it's invalid
Implement Menu widget group

Provides a MenuBar, a popup-ish submenu Menu, MenuButtons,
MenuSeparators. Radio-buttons and check-boxes are yet to come.
Fix missing propagation of kwargs
Merge branch 'ui-widgets' into devel