Activation policy

[AlexPerathoner]

• [x] NSApplication.shared.setActivationPolicy(.accessory) in applicationDidFinishLaunching
• [x] Remove LSUIElement from info.plist
• [x] NSApplication.shared.setActivationPolicy(.regular) after NSApp.activate(ignoringOtherApps: true) (e.g when showing settings window)
• [x] override cmd + q to close window and go to accessory activation policy again
• [x] go to accessory activation policy again also if all windows got closed
• [x] optional: show message once telling user that app won't quit but will continue running in the background
• [x] cmd + w should only close the frontmost window, and go to accessory only if there are no other windows open