New window open logic

[matthme]

This PR changes the logic around creating/opening/showing windows in the following way:

• the settings window is created independently from the main window
• if the settings window is opened from the main window, it will always show next to the main window, i.e. on the same screen
• the CommandOrControl+Shift+L shortcut now also closes the main window if the main window is already focused

[matthme]

replaced by #204