Application activation area cannot be adjusted without using panels

The subject line is perhaps a bit unwieldy, but the general issue is that with the current state of agl-compositor and the agl-shell protocol API it is not possible to control the activation area of applications unless panel surfaces are specified.  This is because the activation area is inherently assumed to be the full size of the background surface unless panels are configured, doing so contrains the area based on the panel size(s).  This proves problematic with single window based UI engines/toolkits like Flutter or web engines, as it is resource intensive to spawn multiple engines for the panels and then have to do extra IPC if making them interact in any way is desired.  A more desirable model is to have a single fullscreen background application with the panel(s) statically present (as always present "chrome") and then have other applications surfaces overlayed in a constrained area (i.e. usually what would be the "background" when using panels, but not necessarily always so).

Based on discussion last week on IRC with @Marius Vlad and on the production IVI EG call, I am opening this and assigning it to myself to cover the prototype that I'm working up to work with the Flutter homescreen.  It is likely that Marius may need to take over at some point to productize my proposed change in line with his plans for agl-compositor and the agl-shell protocol.