Add a slash command to show (or toggle) the main window on demand

[rdw-software]

Source: Discord (deleted message)

i hoped there is a command to show it without needing to use the minimap icon

Not sure how useful this is overall, but it should be pretty easy to do:

1. Add new branch to the slash command handler
2. Show the main window, probably using this code here
3. Now turn it into a toggle, so that if it's run twice the UI is hidden again (?)

Might be a bit slow due to the LibQTip initialization delay, so possibly add some message to indicate the UI is loading?