[applet] Remove config dialog and put options in Hamburger menu
This commit deletes the applet's config dialog and puts its three mutually-exclusive options for what to show into the applet's hamburger menu, exactly like how the Disks & Devices applet does it. This allows removing the action to show the applet's config dialog, which means there is now only a single "configure" action and it opens the KCM. The net result is a UI that is cleaner, less confusing, and more consistent, with the options made more discoverable.
BUG: 407857 FIXED-IN: 21.08