Don't reload whole theme when variant changes on XDG

[Jacalz]

Description:

This is a small improvement in the XDG theme watching code to make sure that we only change the variant and not do a full theme reload each time given that we know that just the variant changed. The delay when switching between dark and light mode is decreased.

Checklist:

• Tests included.
• Lint and formatter run with no errors.
• Tests all pass.

[coveralls]

coverage: 59.957% (-0.001%) from 59.958%
when pulling d515fa8 on Jacalz:theme-change-cleanup
into 85063d5 on fyne-io:develop.

[dweymouth]

Code looks good, I didn't test it myself but I trust that you verified it :)