Buttons: Allow overriding of visual properties when 'action' is set

Those properties are, text, tooltip, iconName, and iconSource.

Also, properly capitalized gallery's tooltips.

Change-Id: I1995a85565edd7aaa420279821d7c1a31ef18253
Reviewed-by: J-P Nurmi <jpnurmi@digia.com>
Reviewed-by: Jens Bache-Wiig <jens.bache-wiig@digia.com>