VSE: Set "box select" default tool for preview and show toolbars

Current default tool is "sample" which is a bit odd, and coupled with
the fact that the toolbar is hidden by default, this can be confusing
to new users, leading them to believe tweak/select tools are not
supported for the VSE preview.

This change:
- Sets "box select" as the default tool for the Preview
- Makes both Preview and Timeline toolbars shown by default

Pull Request: https://projects.blender.org/blender/blender/pulls/126336

source/blender/blenkernel/BKE_blender_version.h

@@ -31,7 +31,7 @@ extern "C" {
 
 /* Blender file format version. */
 #define BLENDER_FILE_VERSION BLENDER_VERSION
-#define BLENDER_FILE_SUBVERSION 19
+#define BLENDER_FILE_SUBVERSION 20
 
 /* Minimum Blender version that supports reading file written with the current
  * version. Older Blender versions will test this and cancel loading the file, showing a warning to

source/blender/blenloader/intern/versioning_400.cc

@@ -69,6 +69,7 @@
 #include "BKE_node_runtime.hh"
 #include "BKE_paint.hh"
 #include "BKE_scene.hh"
+#include "BKE_screen.hh"
 #include "BKE_tracking.h"
 
 #include "IMB_imbuf_enums.h"
@@ -4568,6 +4569,21 @@ void blo_do_versions_400(FileData *fd, Library * /*lib*/, Main *bmain)
     FOREACH_NODETREE_END;
   }
 
+  if (!MAIN_VERSION_FILE_ATLEAST(bmain, 403, 20)) {
+    LISTBASE_FOREACH (bScreen *, screen, &bmain->screens) {
+      LISTBASE_FOREACH (ScrArea *, area, &screen->areabase) {
+        LISTBASE_FOREACH (SpaceLink *, sl, &area->spacedata) {
+          if (sl->spacetype == SPACE_SEQ) {
+            ARegion *region = BKE_area_find_region_type(area, RGN_TYPE_TOOLS);
+            if (region != nullptr) {
+              region->flag &= ~RGN_FLAG_HIDDEN;
+            }
+          }
+        }
+      }
+    }
+  }
+
   /**
    * Always bump subversion in BKE_blender_version.h when adding versioning
    * code here, and wrap it inside a MAIN_VERSION_FILE_ATLEAST check.

source/blender/editors/space_sequencer/space_sequencer.cc

@@ -136,7 +136,6 @@ static SpaceLink *sequencer_create(const ScrArea * /*area*/, const Scene *scene)
   BLI_addtail(&sseq->regionbase, static_cast<void *>(region));
   region->regiontype = RGN_TYPE_TOOLS;
   region->alignment = RGN_ALIGN_LEFT;
-  region->flag = RGN_FLAG_HIDDEN;
 
   /* Channels. */
   region = MEM_cnew<ARegion>("channels for sequencer");

source/blender/windowmanager/intern/wm_toolsystem.cc

@@ -692,7 +692,7 @@ static const char *toolsystem_default_tool(const bToolKey *tkey)
         case SEQ_VIEW_SEQUENCE:
           return "builtin.select";
         case SEQ_VIEW_PREVIEW:
-          return "builtin.sample";
+          return "builtin.select_box";
         case SEQ_VIEW_SEQUENCE_PREVIEW:
           return "builtin.select";
       }