Sort files and directories in a natural way in file trees

Should fix https://github.com/modxcms/revolution/issues/10286

thanks @rainbowtiger for the suggested fix, even he reports that it's not working on his side, in my environment it seems to work nicely...

before:
![bildschirmfoto 2014-07-21 um 19 44 04](https://cloud.githubusercontent.com/assets/445501/3647583/3e2739f6-1100-11e4-8766-2dd5209fce6f.png)

after:
![bildschirmfoto 2014-07-21 um 19 55 53](https://cloud.githubusercontent.com/assets/445501/3647587/4bfe857a-1100-11e4-859a-da45dc86feb4.png)

but don't ask me why the sort order is suddenly messed up slightly for the image_c.png (and variations) files...