UX: Add disabled button

[rugk]

The behaviour of the toolbar button should be improved.

When no text is selected

Current behaviour: Button as usual (enabled)
Best behaviour: Disabled

When text is selected

Current behaviour: Button as usual (enabled)
Best behaviour: Button as usual (enabled)

This behaviour could further be improved by having a floating button. See #95.

[lpagliari]

@rugk This is a good idea, could you help us on that? This hook might be what you need: http://etherpad.org/doc/v1.6.0/#index_aceselectionchanged