Single instance for drop-down mode

[tsujan]

Based on D-Bus.

[marcusbritanicus]

I can confirm that this solves both #1028 and #1059. It should be quite easy to set a (keybind, command) pair to show and toggle the drop-down window on all wayland compositors.

[stefonarch]

Works fine, yes.

This has a nice side-effect: qterminal -d in x11 needs to be in autostart and opens the window at login, while on wayland the first hit on F12 opens it and if you actually close it by ctrl+q no issue, it reopens again at F12. So I added X-LXQt-X11-Only=true in its autostart .desktop file.

[tsujan]

@marcusbritanicus, @stefonarch, thanks for testing it! Will merge it along with #1113 after testing them together for a week or so.

@marcusbritanicus, as I mentioned at #1115, your method was also correct and worked fine. I didn't check why D-Bus was already used in the code, but I didn't find a real use for it by a glance before the current PR.

If needed, we could also make a single-instance app out of the windowed mode later. But I'm not sure if that's a good idea. Personally, I feel safer when my terminal instances are independent from each other. However, the drop-down mode was already used like a single-instance app.

@stefonarch,, please rebase #1113 to fix its conflict (only one) with the current master.

[marcusbritanicus]

@tsujan No issues at all. This way is much shorter and it will be easy on the maintenance. Plus, no new files added.

[luis-pereira]

Testing it.

[tsujan]

This PR and #1113 have worked reliably here for 4 days. I'm merging both...