Fix long-used recent files being truncated out of the list

1.4 introduced a bug that caused entries near the top of recently-used.xbel to be omitted if there were too many entries, even if those files were just opened, instead of the longest-unopened files.

This MR fixes that by sorting the list by date before truncating it to the maximum length, so only the longest-unused files get omitted. Truncation also now happens after duplicates are removed.

This MR also removes instances of #ifdef _WIN32 that aren't present in related code, might cause inconsistent behavior, and are forbidden by the coding conventions.