Make action bar to float when needed

The action bar may be hidden by long list of messages (especially when those are loaded dynamically).
this change makes the action bar to float when it is hidden by scroll so it becomes visible.
When the user reaches the end of the list (or for short list of messages that fit the current viewport), the action bar is positioned at the bottom of the message list.

Change-Id: I148f6ba130efa80cfd3a2bf28c8d4e9467faa435
Adjust z-index for action bar to avoid overlapping with message editor

The z-index has been added since the text area of the message editor
was displaying on top of the action bar while the rest of the editor
was displayed below (in terms of depth). As a result, depth was inconsistent.
with this change, the action bar is displayed on top of the editor when floating.

Change-Id: I91a6e8ed62fa3f7fb55a2a389822d77480a84d65