Properly manage Widget toggling for Collaboration

In collaboration, a user can either be Master or participant.
Any participant is not allowed to edit any proxy property.
But once a participant became Master, all those disabled
Widgets should become enable again.
This is achieved by adding PV_MUST_BE_MASTER
properties to the pqProxyWidget base class.
Then the collaboration behavior will take care of the rest
automatically for you when the user status change.

Change-Id: I73ba7c6fe02dd9a09fe059ad4ccc332c2ebda917
Merge topic 'pv-collaboration-widget-state' into next

ee32b62 Properly manage Widget toggling for Collaboration