Add preliminary single-instance support for drop-down mode

[marcusbritanicus]

This PR adds preliminary support for single instance mode for drop-down mode. Additionally, it adds support for toggling the drop-down instance via qterminal -d command line. On wayland, we can simply add a command-binding pair for most compositors. On wayfire, this would simply be adding the following lines under the [command] section.

binding_qterminal_dd: KEY_F12
command_qterminal_dd: qterminal -d

If an instance of qterminal in the drop-down mode exists, it will toggle it. Otherwise, a new drop-down instance will be created. The normal qterminal instances are unaffected.

This fixes both #1028 and #1059. Since it's not possible to fix one without the other, I am breaking the one-issue-one-PR rule.

Cc: @yan12125 @stefonarch @tsujan

[stefonarch]

Works as it should - for me GTM

[yan12125]

Removing myself as a reviewer as I stopped maintaining qterminal and qtermwidget, see lxqt/lxqt#2564

[tsujan]

I'll review it after QTerminal 2.0 is released.

[tsujan]

Although this code seems OK, I'm afraid not only a D-Bus-based single instance needs much less code (because D-Bus is already used), but also it's more reliable than the socket-lock method (I say that based on my experience with old versions of FeatherPad).

Here is the D-Bus-based PR: #1131

[marcusbritanicus]

@tsujan Just tested #1131. It works great!! I'll close this.

[stefonarch]

Works ok here too, but thanks to @marcusbritanicus 's code which made it already possible to have it for the last weeks ;)