Enable multiple global shortcuts

[JacobMillward]

At the moment you can only set a single global keyboard shortcut to open/close the terminal. It would be useful to be able to set multiple keybinds.

[amezin]

I'm planning to rewrite Preferences to use libadwaita after Gtk 4 port https://github.com/ddterm/gnome-shell-extension-ddterm/issues/652

New Preferences window will include a new shortcut editor.

Until that, you can set multiple shortcuts by editing dconf registry, the process is described in this bug: https://github.com/ddterm/gnome-shell-extension-ddterm/issues/939 (all shortcut values in settings are json-like lists of strings)