fix(developer): Project MRU now saves correctly

Fixes #9899.

Several problems resolved here:
* project mru did not refresh the welcome screen on first load, so the
  welcome screen was often showing projects that were no longer relevant
* content saved to registry did not match expectations on load, so mru
  never worked between sessions (always flushed on reload)
* synchronization between simultaneous sessions did not work, because
  the list was only saved when Keyman Developer exited

Minor tidyup:
* WideString -> string -- legacy cleanup from when Delphi was
  non-Unicode.
* Reordered functions to group more clearly.

Note: the issue was not that there was both registry and xml content;
the source of truth is the registry for Project MRU, and project_mru.xml
is copied from that for the welcome screen. So instead of taking the
fix recommended in #9899, have addressed this by fixing the above points
in the registry code.