Create backup copy

[Francisco Javier Díez]

When saving a modified version of a network,OM should keep the previous version, replacing the extension .pgmx with .bak. (It worked like this in the past, but this behaviour was later lost, I don't know why.)

[Miguel Ángel Artaso Landa]

The functionality was not actually lost but it was being performed only when 'Save and reopen' option was selected from the menu. Now it is also performed when just saving the netwrok. (Commit hash 9318aef330f226f4e991e40660b250c5d8e6ccfb)