Handle-middle-click-24

[colinkiama]

Fixes #24

Note: Changes default behaviour when launched from terminal without command line args to create a new window

[jeremypw]

Need to check how the new elementary dock under development handles middle-click. See also https://github.com/elementary/dock/issues/77#issuecomment-739536949 . This might be an issue that needs to be fixed in dock? It seems to me that dock needs to activate the "New Window" option in the context menu when a middle-click is used. Otherwise there needs to be some way the app knows that a middle-click was used otherwise the behaviour changes in other situations which is undesirable.