Fix stuck menu options on community/room list

[syldrathecat]

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This fixes the issue of checkbox states becoming stuck for the community and room list right-click menus, by putting the Menu instance inside of the ItemDelegate, rather than having them all share a single Menu instance.

[syldrathecat]

Note: This change conflicts with #1062 since it modifies this same menu

[deepbluev7]

I would prefer a different solution. If you use the per item menu instances, scrolling gets quite a bit slower. It could work to pass the model to the menu though as a variable to automatically get updates on property changes.