- Switch to flags instead of boolean argument
- Move logic for layer validity to canRunUsingLayer
- Add unit test
Also remove settings flag to hide duplicate features action
Since there's a number of serious outstanding issues with these
tools, hide them by default and show them only if a QgsSettings
flag is set ("tools\showDuplicateFeatureActions").
This allows the tools to be reworked for 3.2, while still
making them accessible for users who require their functionality.
