Single instance dropdown

[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.

Cc: @yan12125 @stefonarch @tsujan

Edit:
This fixes both #1028 and #1059.

[stefonarch]

Very nice! But we should follow our rules: One issue - one commit/PR otherwise it's hard to keep track. For me single instance + -d option could go together though.

[marcusbritanicus]

I'll close this PR and open a new one. I'm finding it a pain to rebase it on master.