Planning merging the single qqml engine

[Marco Martin]

Hi all,

I think the branches for the single shared qqmlengine are pretty much ready 
(few cleanups upcoming days), seems pretty stable here.. did someone ran the 
branch for a while as well?

In the end i managed to get a single engine for the whole session, views 
included (had to duplicate the View class in plasmaquick and kept the old one 
as deprecated for retrocompatibility unfortunately)

the memory save seems pretty good, i *hope* stability improved as well :p

what still uses separed engines are the applet configuration dialogs: this is 
necessary because they are supposed to use a different style for the 
qquickcontrols, and being the settings object that decides the style a qml 
singleton (qml singletons are unique by engine), the engine has to be 
different from the desktop/panel. The good thing is that config dialogs are 
instantiated relatively rarely, in most sessions not at all, so shouldn't 
touch too much a "normal run"

For retrocompatibility the applets unfortunately have to specify explicitly 
they can share the engine in their metadata file (or eventual plasmapackage:/ 
urls break)

at the moment it's using
X-Plasma-RequiredExtensions=SharedEngine

but I'm leaning more on the direction of something like
X-Plasma-MinimumAPIVersion=5.11

I would like to have all set before frameworks 5.11

thoughts?

-- 
Marco Martin