Reworked the mythfilebrowser window in base.xml.

modified:   base.xml
new file:   images/file_browser_executable_folder.png
new file:   images/file_browser_file_folder.png
new file:   images/file_browser_folder.png
new file:   images/file_browser_no_preview.png
new file:   images/file_browser_up_folder.png
new file:   images/masks/file_browser_preview_mask.png
deleted:    images/masks/myth_file_browser_preview_mask.png