Replace legacy image file format icons with fontawesome in filetree

This PR resolves https://github.com/modxcms/revolution/issues/11851 by replacing the legacy image icons with icons from fontawesome.

It's a compromise for some file formats, as fontawesome doesn't offer a wide variety of icons for different languages, like php, ruby etc., so files containing code now have the same icon in the tree, e.g. the code icon.

I also added additional file formats missing and deserving the right icon (all audio formats, more video formats, archive formats, calendar formats, .svg for images added, .csv, .txt, .docx, .xlsx, .pptx, .sh and more). All currently defined formats you can see in the screencap below:

![modx filetree icons](https://cloud.githubusercontent.com/assets/445501/3799658/ccd4a2e8-1bef-11e4-913c-f4b92e33369e.gif)

if some format is missing or should have a different icon, please comment!