RFC: Hide less important popups on new modals

Right now we don't have any prioritizing in place to hide shell popups in case something more important comes up, this allows for bugs like #461 (closed) to happen where things like the alt-tab switcher remain opened while the shell is showing a modal dialog or the overview.

A pretty effective to fix this is to hide switcher popups and popup menus (maybe the pad OSD, too?) as soon as a new modal is pushed.

I'm not very happy with shell_global_emit_modal_pushed though and would prefer to move the whole handling of modals from main.js to shell-global. Also I'm not sure how to handle modal dialogs, for those it might be a good idea to prevent opening new ones if another modal dialog is already visible.