Open an external photo editor from GUI

[pmllc]

I think that it could be very usefull to be able to launch directly from HUGIN interface a photo editor without using external file browser.
For example, when you have to add alpha layer on files, it should be easier to identitfy the file from HUGIN interface. firt step should be on picture tabs, a very powerfull one (no idea on implementation) should be on preview screen;
My english is not very good so do not hesitate for clarification.

Another time, very good job.