• Control+shift+groupKey already adds the selected units into the group
• alt+groupKey now removes the units from all the groups and adds them into the group
• control+alt+groupKey now removes the units from all the groups and creates a new group with them

the controlscheme was taken from Starcraft 2

partially fixes #223 (there's no hotkey to only remove, but I think that alt is good enough)

Improvements:

• Honestly Control+shift+groupKey is hard to press, I'd change it to shift+groupKey
• the modifiers should be customizable instead of being hardcoded (Add the option to use key combos as hotkeys. #230 )