Moved Focusable interface from AnchorButton to AbstractButton base class for use with all button types.
Improved FocusableMixin, removed obsolete HasTabIndex interface and TabIndexMixin.

Conflicts:
	gwtbootstrap3/src/main/java/org/gwtbootstrap3/client/ui/Anchor.java
	gwtbootstrap3/src/main/java/org/gwtbootstrap3/client/ui/AnchorButton.java
	gwtbootstrap3/src/main/java/org/gwtbootstrap3/client/ui/HasTabIndex.java
	gwtbootstrap3/src/main/java/org/gwtbootstrap3/client/ui/base/button/AbstractButton.java