Allow opening a graph file by double clicking

[blsemo]

When double clicking a graph file in Windows Explorer (windows), and probably similar in Finder on Mac, it should be possible to associate depthmapX with the file type so it opens it.

[pklampros]

For macOS a double clicked file will (after this pull request) be opened in depthmapX if the application is not already open. If depthmapX is already open then the new file will not be opened and needs to be opened from within the application