Fix mouse interactions with tabwin buttons

Signed-off-by: Simon Steinbeiss <simon.steinbeiss@elfenbeinturm.at>
Account for custom font sizes in the tabwin label

Signed-off-by: Simon Steinbeiss <simon.steinbeiss@elfenbeinturm.at>
Install a new style property "listview-icon-size"
themes can add "Xfwm4TabwinWidget::listview-icon-size = $n" to set the appicon-size in the tabwin's new listview. Currently min=16 and max=48
Merge remote-tracking branch 'origin/ochosi/tabwin'