Normal clicking the launcher icon just brings the application to
the foreground so there must be a way users can create a new
instance of Geany.

Add an entry "New Window" to the context menu which is shown
when right-clicking the Geany icon in the launcher (most applications
have the "New Window" entry there).

In addition, fix "Open in new window" when using app bundle.

Since both of these functionalities create a new Geany instance,
factor-out the instance creation code into a new utility function
and use it in both cases.