Asset System: Prepare File Browser backend for the Asset Browser

The Asset Browser will be a sub-editor of the File Browser. This prepares the
File Browser code for that.

**File-Lists**
* Support loading assets with metadata read from external files into the
  file-list.
* New main based file-list type, for the "Current File" asset library.
* Refresh file-list when switching between browse modes or asset libraries.
* Support empty file-lists (asset library with no assets).
* Store file previews as icons, so scripts can reference them via icon-id. See
  previous commit.

**Space Data**
* Introduce "browse mode" to differeniate between file and asset browsing.
* Add `FileAssetSelectParams` to `SpaceFile`, with `FileSelectParams` as base.
  Makes sure data is separated between asset and file browsing when switching
  between them. The active params can be obtained through
  `ED_fileselect_get_active_params()`.
* `FileAssetSelectParams` stores the currently visible asset library ID.
* Introduce file history abstraction so file and asset browsing can keep a
  separate history (previous and next directories).

**General**
* Option to only show asset data-blocks while file browsing (not exposed here).
* Add "active_file" context member, so scripts can get and display info about
  the active file.
* Add "active_id" context member, so `ED_OT_lib_id_load_custom_preview` can set
  a custom ID preview. (Only for "Current File" asset library)
* Expose some of `FileDirEntry` in RNA as (non-editable). That way scripts can
  obtain name, preview icon and asset-data.

Part of the first Asset Browser milestone. Check the #asset_browser_milestone_1
project milestone on developer.blender.org.

Differential Revision: https://developer.blender.org/D9724

Reviewed by: Bastien Montagne

1 files changed, 10 insertions, 1 deletions

diff --git a/source/blender/blenkernel/intern/screen.c b/source/blender/blenkernel/intern/screen.c
index ae817c46931..52c41c9fd05 100644
--- a/source/blender/blenkernel/intern/screen.c
+++ b/source/blender/blenkernel/intern/screen.c
@@ -1266,6 +1266,9 @@ static void write_area_regions(BlendWriter *writer, ScrArea *area)
       if (sfile->params) {
         BLO_write_struct(writer, FileSelectParams, sfile->params);
       }
+      if (sfile->asset_params) {
+        BLO_write_struct(writer, FileAssetSelectParams, sfile->asset_params);
+      }
     }
     else if (sl->spacetype == SPACE_SEQ) {
       BLO_write_struct(writer, SpaceSeq, sl);
@@ -1663,11 +1666,14 @@ static void direct_link_area(BlendDataReader *reader, ScrArea *area)
        * plus, it isn't saved to files yet!
        */
       sfile->folders_prev = sfile->folders_next = NULL;
+      BLI_listbase_clear(&sfile->folder_histories);
       sfile->files = NULL;
       sfile->layout = NULL;
       sfile->op = NULL;
       sfile->previews_timer = NULL;
+      sfile->tags = 0;
       BLO_read_data_address(reader, &sfile->params);
+      BLO_read_data_address(reader, &sfile->asset_params);
     }
     else if (sl->spacetype == SPACE_CLIP) {
       SpaceClip *sclip = (SpaceClip *)sl;
@@ -1751,8 +1757,11 @@ void BKE_screen_area_blend_read_lib(BlendLibReader *reader, ID *parent_id, ScrAr
         }
         break;
       }
-      case SPACE_FILE:
+      case SPACE_FILE: {
+        SpaceFile *sfile = (SpaceFile *)sl;
+        sfile->tags |= FILE_TAG_REBUILD_MAIN_FILES;
         break;
+      }
       case SPACE_ACTION: {
         SpaceAction *saction = (SpaceAction *)sl;
         bDopeSheet *ads = &saction->ads;