Use QApplication::activeWindow() instead of QApplication::topLevelWidgets() to retrieve current screen

[nyalldawson]

QApplication::topLevelWidgets() can be EXTREMELY costly to call,
as it builds a list dynamically containing potentially dozens/hundreds
of widgets.

Fixes slow interaction with the layer tree in large projects

(master only)

[nyalldawson]

Ping @m-kuhn

[m-kuhn]

Looks good to me.
Or better: "not worse than what we always have been doing"
Ideally we'd pass in a context for where this will be actually rendered, but that's a bigger undertaking of a general review of the hidpi code (not sure if qt did any advance here).

[nyalldawson]

Ideally we'd pass in a context for where this will be actually rendered, but that's a bigger undertaking of a general review of the hidpi code (not sure if qt did any advance here).

Honestly i'm not sure it's possible in qt models -- it's a big shortcoming where the view can't pass any context back to the data call , so we can't make a model which adapts to hidpi/non-hdpi view widgets on demand...

[m-kuhn]

Exactly where I also landed ...
The best I could come up with is QgsXyzModel::setTargetDpi() and invalidate data when it changes.