ES5 → 7: Use enhanced object properties

http://es6-features.org/#PropertyShorthand

Instead of doing {"foo": foo, "bar": bar, ...}, we can just do
{foo, bar} now.

The function parameters of EventHandlers have all been renamed to
camelCase to make use of this, as the JS style conventions intend.
Other functions will follow in a later commit.

src/qml/Chat/Banners/LeftBanner.qml

@@ -22,13 +22,13 @@ Banner {
         }
     ]
 
-    buttonCallbacks: {
-        "forget": button => {
+    buttonCallbacks: ({
+        forget: button => {
             button.loading = true
             py.callClientCoro(
                 chatPage.userId, "room_forget", [chatPage.roomId], () => {
                     button.loading = false
             })
         }
-    }
+    })
 }