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.

TODO.md

@@ -10,7 +10,6 @@
 -  `QCoreApplication::setAttribute(Qt::AA_EnableHighDpiScaling)` ?
 
 - Qt 5.12
-  - See about ECMAScript 6 and 7 features
   - .mjs modules
 
 - Refactoring
@@ -18,6 +17,7 @@
   - Unfinished work in button-refactor branch
     - Button can get "hoverEnabled: false" to let HoverHandlers work
   - Room Sidepane
+  - When qml syntax highlighting supports string interpolation, use them
 
 - Bug fixes
   - Past events loading (limit 100) freezes the GUI - need to move upsert func