Rework MMFromRelationEditor

[tomasMizera]

Also adds an additional mouse area - when a user clicks on the white background, it opens the features list. It was behaving like this even before, but we had forgotten about that in the new implementation.

[github-actions]

Pull Request Test Coverage Report for Build 8740045044

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 8 unchanged lines in 2 files lost coverage.
• Overall coverage decreased (-0.06%) to 59.997%

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Files with Coverage Reduction	New Missed Lines	%
input/core/merginuserinfo.cpp	3	72.99%
input/app/projectsmodel.cpp	5	65.75%

Totals
Change from base Build 8739644591:	-0.06%
Covered Lines:	7577
Relevant Lines:	12629

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💛 - Coveralls