skins2: implement a new 'focus' attribute for text control

Up to now, text control could but be focusable, and this focus was used
to scroll text, should it be too long for full display.

From a user viewpoint, this behavior is often counterintuitive :
 - Most of the time, the control is designed to fit the text anyway, and
   scrolling is not expected.
 - On the other hand, an underlying control may be used to move the window
   (for instance, the title bar), and a text control is expected not to
   interact in this endeavor. Same goes for popupmenu, that is expected
   to work if one happens to right-click on a text control.

This patch adds a 'focus' attribute to the text control, and leave it to the
skins developper to decide which behavior is most desirable on a per-control
basis. focus still defaults to true to maintain skins current behavior.