[5.x] FR: Add source option to "Only show in element selector modals"

[thupsi]

Sometimes we create custom sources that only serve as "entry selection sources".

We can set these sources to only be visible for certain user groups, but still they add noise for the admin.

I would like to propose a new "Only show in element selector modals" source setting which would hide the source for all users (even for admins) but still allow entry selection in entries fields.

Another way to go about it would be to add "Admin" to the User Groups source setting (though I still prefer the first solution):

Thanks for considering!