New: Allow key area width to be overriden through style
New: Allow key area width to be overriden through style

RevBy: Kimmo Surakka, Pekka Vuorela
Details: So far, we let QGraphicsLayouts manage the width of key areas. When
not managed by a layout, the key area needs manual resizing. This commit allows to
constrain the maximum width through MImAbstractKeyAreaStyle's size attribute
(in CSS file).
I feel that the right way to fix this would have been a proper sizeHint
implementation, but I couldn't get that to work.

RHawkeyed/plugins,RHawkeyed/plugins,develersrl/maliit-demo-qml,develersrl/maliit-demo-qml,KDE/plasma-maliit-plugins,RHawkeyed/plugins,KDE/plasma-maliit-plugins,KDE/plasma-maliit-plugins,develersrl/maliit-demo-qml,RHawkeyed/plugins,develersrl/maliit-demo-qml,RHawkeyed/plugins,maliit/plugins,KDE/plasma-maliit-plugins,maliit/plugins,maliit/plugins,maliit/plugins