Preference Packs: Add revert to backup option

[chennes]

Adds an option to the Preference Pack GUI to revert to a saved preference file.

When a preference pack is applied, FreeCAD automatically backs up the pre-pack config file. This PR adds a GUI to reverting to one of those saved backups. Right now this is a somewhat destructive process: the backup is a complete copy of the original config file, including things like "Recent Files". Reverting to the backup will thus reset your Recent Files list to whatever it was when the backup was taken.

In the future it would be best to make a more judicious backup, that only backs up the items that the preference pack affects. I submit this PR as an improvement over the current state of affairs, not as the end goal.

Testers: please make a manual backup of your user.cfg file prior to testing. To test, apply a preference pack (any one will do), then click the "Revert" button.

[freecadci]

for feature branch PR_6339. Pipeline 468687074 was triggered at 704fad4. All CI branches and pipelines.

[freecadci]

for feature branch PR_6339. Pipeline 469288401 was triggered at dea725d. All CI branches and pipelines.

[freecadci]

for feature branch PR_6339. Pipeline 469448448 was triggered at 0dc7cff. All CI branches and pipelines.

[freecadci]

for feature branch PR_6339. Pipeline 470452288 was triggered at 5458da6. All CI branches and pipelines.

[freecadci]

for feature branch PR_6339. Pipeline 474609625 was triggered at b95343a. All CI branches and pipelines.