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.