Only relying on the .asset.blend extension was weak, it was too easy to save
files this way. Not just by manually naming files, but also by using "Save As"
on such a file (as suggested by previous the warning popup), without actually
changing the file name.

Instead, additionally set a flag for asset system files, only treat these as
writable by the asset system, and disallow overriding them manually.

- Opening an asset system file shows a warning.
- The status bar shows a permanent warning for such a file.
- Attempting to save such an asset system file will fail with a warning popup.
  It lets you choose to cancel or use "Save As" instead, but not override the
  file.
- When using "Save As" from an asset system file, the .asset.blend is replaced
  with .blend
- An additional check in the main .blend file writing function prevents
  writing files as asset system files if they are not identified as such.

This should prevent us from any unintentional user data loss through file
saving and asset editing.

Files may still manually be named .asset.blend now. They are just normal
.blend files.

@@ -297,0 +299,4 @@
+   * UI, at which point the file will be regenerated by the asset system (API in
+   * #BKE_asset_edit.hh). Stored with a .asset.blend prefix.
+   */
+  G_FILE_MINIMAL_ASSET_EDIT_FILE = (1 << 29),

@@ -458,6 +462,9 @@ static void read_file_version(FileData *fd, Main *main)
   if (main->curlib) {
     main->curlib->runtime.versionfile = main->versionfile;
     main->curlib->runtime.subversionfile = main->subversionfile;
+    if (main->is_asset_edit_file) {

@@ -553,0 +554,4 @@
+   * The blend file of this library has the #G_FILE_MINIMAL_ASSET_EDIT_FILE flag set (refer to it
+   * for more info).
+   */
+  LIBRARY_IS_MINIMAL_ASSET_EDIT_FILE = 1 << 3,