toolbar: Provide a visual cue when find is unsupported
When document type does not support 'find', Evince disables the find feature. Previously, we added a tooltip explaining the reason. However, a straightforward visual cue is more desirable, especially for touchpad users.
Aside the tooltip, we now also provide an icon that mimic other icons in GNOME when a service is unavailable, that is, an 'x' on the icon for mute or wifi.
Fixes #105 (closed)