Defer mask source widget population (fixes #34942)

[mhugo]

Description

Defer the loading of masking widgets (mask sources and targets) in the layer properties.

They are loaded only on explicit user request, when the user ticks a checkbox.

Should fix #34942

[Replace this with some text explaining the rationale and details about this pull request]

[nyalldawson]

Fix looks good! Can you fix the widget alignment in the panel though? (label vs treeview). The layout would look nicer (and more consistent with the other tabs) if you used a checkable group box here instead of the seperate qcheckbox/qframe approach.

[SteveLowman]

Does this mean the merge has been completed?

[mhugo]

@nyalldawson Yes indeed, thanks for the reminder. That should be better now.

@SteveLowman Not yet, I'll merge once the automatic tests are ok and will tell you.

[SteveLowman]

@mhugo I downloaded and installed Friday's nightly build, but the fix does not seem to be there. Should I still be waiting?

[mhugo]

@SteveLowman Well it should be there in the "qgis-dev" version (3.13.0-27) and it should now also be in the "qgis-rel-dev" (3.12.1-10) that can be installed through the OSGeo4W installer