Improve alternate parsing

[qmfrederik]

There are at least two bugs in alternate parsing:

• On all platforms, the last value in the alternate files is ignored
• On Windows, the colon in full paths (e.g. C:\Users\Me) is used as a separator

This PR attempts to fix both issues. Contributes towards #595

[AArnott]

Looks good. A couple quick things to consider.

[AArnott]

@qmfrederik If you push to this PR again, keep in mind I rebased your commits to allow for targeting the fix to v3.4. You should do a fetch and git reset --hard origin/features/alternates before doing more work to avoid inadvertently merging master into v3.4.