Currently, in master, when saving over an old preset, the new preset is saved to presetNameGBLGK.kpp with the help of QTemporaryFile, and the oldpreset is blacklisted.
This patch does two things.

1. The new preset will be saved using an unmodified name, while the old preset is given the new file name and blacklisted. This means that the latest version of the preset is always named presetName.kpp
2. The renamed filename is turned into presetName_backup_datetimestamp.kpp

These two things should make making presets a lot more user-friendly. This fixes https://bugs.kde.org/show_bug.cgi?id=330251

I have veriefied this works for pre-installed preset(that is, presets that are loaded from the install directory, and not the resource directory). These require extra help to make sure that they get blacklisted, so I am happy with that.

However, as you may know everything that happens in Kis_Paintop_Box.cc is bound to haunt us for the coming year, so I refuse to commit this patch unless all the reviewers I assigned have tested and accepted the thing.