Draft: Handle open/retract shortcut via DBus activation

Currently we handle the open/retract shortcut by binding it to the relevant QAction.

This has two drawbacks:

• We can only handle the shortcut when Yakuake is running and can't make Yakuake start on demand when the shortcut is pressed. https://bugs.kde.org/show_bug.cgi?id=465263
• On Wayland we don't get an activation token when the shortcut is pressed and can't raise existing windows as a result. BUG:402634 BUG:381541

We can address both of these things by porting the shortcut handling to use DBus activation. When Yakuake is not running and the shortcut is pressed it will be started. When it is running already and the shortcut is pressed it is activated and we call toggleWindowState. We get a startupId/activation token from KGlobalAccel that we can use to raise existing windows

This needs frameworks/kio!1580 (merged) to work

Draft because:

• Existing non-default shortcuts need to be migrated
• We display the current shortcut in various places in the UI. That code needs to be adapted
• Retracting causes an unwanted startup feedback in the panel. That probably needs some handling of StartupNotify=false in Plasma/KWin
• Changing the shortcut in systemsettings doesn't update the UI
• The shortcut cannot be configured in the local shortcuts settings any more
• The shortcut cannot be configured from the first run wizard