Add SingleInstanceMode to Photino initialization

[philippjbauer]

In order to prevent the user from opening multiple instances of a Photino app we need to implement a check during initialization that checks if another instance is running. An optional, configurable message should be displayed in a native warning dialog if the user tries to open a second instance.

[wthks]

Please make this configurable, at the moment you can't even open 2 different files that run with photino.

[ottodobretsberger]

This is on our backlog of bugs and features to implement.