Added more flexibility in *.kv filenames

I think there should be a little more flexibility in kv filenames.

This patch allows for:
* Adding *app (Upper- or Lowercase) in kv filename (Example: TestApp.kv and Test.kv both work. If they both exist, the former will be used.)
* Lowercase *app classname suffixes.

I also moved the .lower() to avoid repetition.