application: Show the window right
If we have at least one active call, show the call window, else show the main window.
This isn't necessarily a complete patch, but should be good enough. As it was said that there will be two separate windows (https://source.puri.sm/Librem5/calls/merge_requests/18#note_7531), we have to make exceptions when showing windows.
The following cases are fixed in this patch:
- If the user closed the main window, create one if required and show that
- if the call window has at least one active call, show the calls window.