Add right click context menus

[clragon]

Is your feature request related to a problem? Please describe.
For desktop clients, there are no right click menus, which makes it feel less desktop.

Describe the solution you'd like
We should add context menus on right click and on long click.
Those menus should be synced up with the three-dot-menu that might be available in the vicinity.

Additional context
We can use flutter_context_menu which I have explored already and works well.

In addition, we can use some kind of inherited widget construct to declare an Area of menu items which can then be translated to either right click, long press or three dot menu.

We also have to consider how this will work with tags and other long-click actions at the moment.