Add fileselect filepath validation check

When the filename field loses focus it will now be checked for validity.
Filenames with no or the wrong suffix will be corrected. If the filename
is cleared, it will be reset to the initial value when the fileselect
was invoked.

Fixes #7.

io_export_o3d/__init__.py

@@ -162,13 +162,33 @@ def invoke(self, context: Context, _: Event) -> set[str]:
             # change stem
             path = path.with_stem(stem)
 
+        # store initial name for use later
+        self.initial_name = path.name
+
         # set filepath
         self.filepath = str(path)
         # show the file selector
         context.window_manager.fileselect_add(self)
         # prevent the operator terminating while in the file selector
         return {'RUNNING_MODAL'}
 
+    def check(self, _: Context) -> bool:
+        path = Path(self.filepath)
+        # check if user cleared the filename box
+        if path.is_dir():
+            # reset filename to initial value
+            path /= self.initial_name
+        # ensure suffix
+        path = path.with_suffix(self.filename_ext)
+        # check for differences
+        if path == Path(self.filepath):
+            # nothing to redraw
+            return False
+        # set filepath
+        self.filepath = str(path)
+        # change to redraw
+        return True
+
     def draw(self, _: Context) -> None:
         pass