2.5 filebrowser: previous/next + bugfix + elubie's changes and cleanup

2.5 filebrowser: previous/next + bugfix + elubie's changes and cleanup
* Previous/Next Folder browser
* bugfix: "open most recently opened directory".

* Previous and Next functionalities:
- use BACKSPACE to navigate to previous folders
- use SHIFT+BACKSPACE to navigate forward
- once you change the folder by other ways the forward folder list is cleared

* bug fix: the sfile->params->dir set through ED_fileselect_set_params wasn't correct. According to the code taking the settings from the existing (previous) filebrowser is a temp solution. In that case this is a fix for a temp solution :)
(changes in: wm_event_system.c, filesel.c and ED_fileselect.h)

** Andrea(elubie): we can get away of the folderlist_clear_next test if we manually pass a boolean to file_change_dir (e.g. file_change_dir(sfile, true)). I tried not to mess up with your changes here. It's slightly slower (and maybe hacky) but its's more conservative IMHO.

(my first commit to 2.5 ... that was a good reason to put my paper on hold :p)