Add MenuStyle::font

Change-Id: I3f4da2593b76b5c5446c1a63df852cc715e56972
Reviewed-by: Caroline Chao <caroline.chao@digia.com>
Add ComboBoxStyle::font

Replace the hidden editorFont-property (recently added for the Android
style) with a proper font-property that is used in other styles too.
The same font is used for the label and the editor.

Change-Id: I3fd1aa1d501ada335046c0d0401119e2e0a22c11
Reviewed-by: Caroline Chao <caroline.chao@digia.com>