Add framework for settings backwards compatibility

[fwdekker]

Fixes #464.

Stores the plugin's version in the user's settings to allow version detection in the future. Furthermore, though Settings remains the usual object with a templateList (and perhaps other fields in the future (cf. #469?)), the class PersistentSettings now acts as if settings are saved as an Element (i.e. as XML). When settings are loaded, PersistentSettings has the opportunity to modify the XML before deserialising it into a Settings. Therefore, if there are incompatibilities between versions that the default XML deserialiser cannot handle, a small bit of code can be added to make these modifications automatically.

[codecov]

Codecov Report

Merging #477 (6c25739) into main (0ccf143) will decrease coverage by 0.34%.
The diff coverage is 56.52%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #477      +/-   ##
==========================================
- Coverage   85.27%   84.93%   -0.34%     
==========================================
  Files          50       51       +1     
  Lines        1881     1899      +18     
  Branches      317      328      +11     
==========================================
+ Hits         1604     1613       +9     
- Misses        216      225       +9     
  Partials       61       61              

[fwdekker]

Decreased coverage is likely due to #upgrade not being tested. Which makes sense, because the code in there is intentionally placeholder dead code. But it's very likely that it'll be used as of v3.1.0.