GrepDialog.py's findfiles() method lacks a unit test.

The comments in the unit test stub in test_grep.py correctly notes that since findfiles() method does not rely on the GrepDialog class, it can be made into a function.

The attached patch does the following:

- Moves findfiles() to be a function in the GrepDialog.py module.
- Adds a unit test for findfiles()
- Adds sensible default arguments
- As suggested by the previous stub comments, findfiles() returns a generator instead of a full list
- Changes 'list' and 'dir' names since those are built-ins
- Uses os.walk() instead of reinventing it.

There were so many changes to make that I eventually just rewrote the entire findfiles function. I've checked that the new version returns the same strings as the old version.