grabs/moving: cleanup drag in tiling layer if window doesn't exist

[ids1024]

It looks like #1475 isn't happening after this, but I need to test a bit more since it's a little hard to reproduce consistently.

This is also meant to fix an issue that can be seen by closing a window (for instance exit in cosmic-term, or super+q) while dragging it still leaving a placeholder in the tiling layout. I'm still not entirely sure why that wasn't causing a crash, but dbeaver was.

This may not be exactly the right way to fix this. I've also noticed an issue where if an output doesn't have any windows, a placeholder isn't created when dragging a window to that output.

[Drakulix]

Seems fine to me

[ids1024]

I guess I didn't get around to looking for better solutions here or adding more fixes to the PR.

But this should be working fine anyway. I guess if @Drakulix thinks it looks fine, we can merge it, and improve the tiling shell code more in later PRs.

• The first commit here fixes the crash with DBeaver, demonstrated in the video on [BUG] cosmic-comp crashes when attempting to stack DBeaver while it's launching #1475. It seems pretty easy to reproduce with DBeaver if you follow the right steps.
• The second commit fixes a a visual issue with dragging tiling windows.
  • Previously, on dragging to an output with no windows on it, no placeholder would be created to indicate the drag. While a placeholder would be crated to show where the window would be dragged to on the output it was dragged from, but would disappear but not reappear and moving to another output and back.
  • Now it adds placeholders as expected for workspaces with no windows on them.

[leviport]

Both fixes are working and I'm not seeing any regressions.