Specify encoding for thumbnails path

Fixes an issue that list view miniatures of the imported clips not
rendered if path contains non-English letters.

By defult the ".text" field in response uses ISO-8859-1 charset (RFC
2616 specs) while server sends utf-8 encoded path string just few
strings below in the code.

src/classes/thumbnail.py

@@ -153,7 +153,7 @@ def do_GET(self):
         if not only_path:
             self.send_header('Content-type', 'image/png')
         else:
-            self.send_header('Content-type', 'text/html')
+            self.send_header('Content-type', 'text/html; charset=utf-8')
         self.end_headers()
 
         # Locate thumbnail