add path of source into name of backup file

[swhib]

with this patch C:\Users\username\Desktop\test.odt should be saved with this name: C__Users_username_Desktop___test___20180325_213617.odt

my motivation: I have a folder per project/client. My filenames are quite often only the current date. I often write multiple letters/documents per day. With your current implementation it was difficult to distinguish the source files for each backup file.

I don't know how useful this is to other people. I opened this pull request in case you consider this to be of general interest.

additional info:

• I use dirname=ReplaceString(... to remove : from paths in windows.
• Tested on win10 and linux

[kovleventer]

Right now I will leave this pr open, since this is a subjective matter and highly depends on workstyle. Counterexample: if both the backup path and the working directory path had a long name, it would cause errors by going over the Windows full path length limit, and this might be the case for some people.

If the majority of people need this feature, I'll add it to master. Perhaps the best solution would be introducing new menu elements or settings functionality, to pick from these options.