2.5 filebrowser

2.5 filebrowser
Bugfixes:
* crash when loading file that has filebrowser open
* file size over 2GB/4GB? shows negative number
Other:
* tried to improve drawing speed a bit by only drawing the files actually shown and improving refresh behaviour a bit.

Note: Solution I found so far is to use the non-standard _stat64, as far as I could see, WIN64 should work without that patch (Genscher, please check and if needed you can enable this too by removing the !defined(WIN64) )
This probably needs to be fixed in at least one other place (BLI_filesize), will look into this once this fix proves stable enough)