Provide feedback for slow inline image paste

As discussed this adds a pulsing progress bar above the composer input upon the slow operations which can result if pasting a large inline image.

Have changed the PNG encoding to use a stream instead which appears to prevent (main?) thread blocking.

Marked WIP as I was wondering..

• If there's an easy way to get rid of the pixel gap above the progress bar? I looked at putting the GtkOverlay around the GtkFrame but was having some initial trouble so thought some feedback might help on whether that's the right path anyway.
• Whether it might be best to centralise the timeout and pulse consts and share them with the ConversationMessage?

Closes #640 (closed)