Final 'FileBrowser First Stage' merge.

It basically rewrites most of filelist.c, with some more limited changes in other areas of filebrowser.

From user perspective, it:
* Removes some info in 'long' drawing mode (owner, permissions) - OS-specific data that do not really matter in Blender!
* Makes short/long display 'fixed' size (among four choices, like thumbnails mode).
* Allows to list several layers of dirtree at once, in a flat way (inside .blend files and/or real directories).
* Consequently, adds datablocks types filtering.
* Uses way less RAM when listing big directories, especially in thumbnail mode (we are talking of several hundred of MiB spared).
* Generates thumbnails way faster.

From code perspective, it:
* Is ready for asset engine needs (on data structure level in filebrowser's listing).
* Simplifies and makes 'generic' file listing much lighter.
* Separates file listing in three different aspects:
** 'generic' filelisting (in BLI), which becomes a shallow wrapper around stat struct.
** 'filebrowser drawing' filelisting, which only contains current visible subset of the whole list (sliding window), with extra drawing data (strings for size, date/time, preview, etc.).
** 'asset-ready' filelisting, which is used for operations common to 'basic' filehandling and future asset-related one.
* Uses uuid's to handle file selection/state in the browser, instead of using flags in filelisting items.
* Uses much lighter BLI_task handling for previews, instead of heavy 'job' system (using the new 'notifier' timer to handle UI refresh, in similar way to jobs).
* Moves .blend datablocks preview handling to IMB_thumbnail (necessary to avoid storing all datablock previews at once, and gives better consistency and performances too).

Revision: https://developer.blender.org/D1316

Thanks to Campbell & Sergey for the reviews. :)

source/blender/editors/space_file/file_intern.h

@@ -48,6 +48,8 @@ struct ARegion *file_tools_region(struct ScrArea *sa);
 #define IMASEL_BUTTONS_HEIGHT (UI_UNIT_Y * 2)
 #define IMASEL_BUTTONS_MARGIN (UI_UNIT_Y / 6)
 
+#define SMALL_SIZE_CHECK(_size) ((_size) < 64)  /* Related to FileSelectParams.thumbnail_size. */
+
 void file_draw_buttons(const bContext *C, ARegion *ar);
 void file_calc_previews(const bContext *C, ARegion *ar);
 void file_draw_list(const bContext *C, ARegion *ar);