Add mapping for volume on Shift+Plus

After this suggestion:

https://forums.plex.tv/discussion/comment/1214893/#Comment_1214893

(Ideally, we should try to see if we can get something better from Qt,
i.e. make it remove the Shift part, but for now this will do.)

resources/inputmaps/keyboard.json

@@ -83,7 +83,7 @@
 
     // volume
     "=": "increase_volume",
-    "\\+": "increase_volume",
+    "(Shift\\+)?\\+": "increase_volume",
     "\\-": "decrease_volume",
 
     // debug command to crash the host. for testing