Add a `X-Plasma-Can-Be-Windowed` option to applets
Review Request #122502 - Created Feb. 9, 2015 and submitted
We can use this option to figure out which applets should be exposed as
windows. It doesn't make sense to do that for all of them.
Tried by using this option to filter them in the windowed widgets runner.
a bit on the fence about it, hmm..
I assume it would just not list them in krunner, but plasma-windowed would still load them from commandline
Anyways, I would call it X-Plasma-StandAloneApp or a formulation like that, then it mirrors nicely the similar current entry X-Plasma-NotificationArea used for the ones that go in the systray