Show pending state on action button and keep dialog open when switching to source mode

Instead of doing a blocking overlay, we're simply keeping the dialog open,
which is necessary for the pending status of the action buttons anyway.

Requires Ib2c8f336 in OOUI

Bug: 65012
Change-Id: I65b5de4a1666a81b157a71f6fec490007689eb44
Merge "Show pending state on action button and keep dialog open when switching to source mode"