Fix missing margins and incorrect size for ToolBar and StatusBar

Using the implicitHeight of an item makes ToolBar and StatusBar
a lot more useable with layouts. In addition we enable the padding
property from the style, allowing its content to be positioned
in a more convenient way. This is similar to how we already do it
for GroupBox.

Change-Id: Ia2aa1bcb9ae109718fd752401400222adec242ac
Reviewed-by: Gabriel de Dietrich <gabriel.dedietrich@digia.com>