Installing 5.1.x after 5.2.x will no longer override config/data locations

[hgohel]

Fix #13261

In an existing 5.2.x installation with XDG directory layout for data/config/cache, installing 5.1.x later would create a second config folder in the legacy layout, overriding the existing config.

This change makes it so that an existing configuration for 5.2.x will be given preference over the 5.1.x config. If there isn't one, previous logic will continue to be used.

Testing passed on Windows

1. Gramps 5.2.2 only
2. Gramps 5.2.2 followed by Gramps 5.1.6
3. Gramps 5.1.6 followed by Gramps 5.2.2

• Create family trees in each version of Gramps and ensure that they are available when that version is launched.
• Ensure that if 5.1.x is installed before 5.2.x, config is imported.

• [X] Testing on Windows
• [x] Testing on Linux
• [X] Testing on MacOS
• [ ] Testing with GRAMPSHOME and SAFEMODE (cli?).

Thanks!

[jralls]

macOS, at least in the AIO bundle, isn't affected by the bug and this change is harmless.

[hgohel]

macOS, at least in the AIO bundle, isn't affected by the bug and this change is harmless.

Thanks for reviewing the change.

[hgohel]

@Nick-Hall Returning to this defect after a while. Discovered a related bug while trying out safe mode, which affects the testing of this one so I think it needs to be fixed first: 13300. It would be very convenient if the root cause of that turns out to be the same as this one.

[hgohel]

@Nick-Hall Turns out that this defect is a side-effect of 13300, and the fix for that is in PR #1726 (gramps52) and PR #1725 (master). I'm closing this PR in favor of those.