Rework source path parsing.

- As we parse the image's function list, we now track the last source
file we encountered. If it's the first time we encounter the current
file, we parse its source path components up front and then simply walk
the parsed list in order to add the function to its appropriate place in
the model, rather than the previous recursive approach.  This allows us
to reuse the parsed component list for subsequent functions in the same
source file rather than having to reparse the path on every iteration.

- Refactor GetFunctionPath() to make use of the new
_GetSourcePathComponents() parsing function.

Should further improve the time needed to change the active image.