Fix context menu positioning for real

[gbalint]

This fix was just a temporary hack, fix it properly:

#5613

This bug affects other context menus using MenuProvider (e.g. dependency list items context menu).
It is probably enough to follow the patterns of the context menus which work well, e.g. the insert/replace menu in the navigator.

[maltenuhn]

It is probably enough to follow the patterns of the context menus which work well, e.g. the insert/replace menu in the navigator.
for the note: that menu does work better, but does not work well either, because it's positioned relative to the mouse position, not the button... We will need a new component for menus contextual to an element (no idea what to call that)