Remove some hacks and unconventional things

InstallApplicationButton: Clean up property access and formatting

ApplicationDelegate: Turn application property into required

Removes some hacks at the expense of not using an alias.

Rename busy properties to be more conventional

Boolean properties are often prefixed with is in their getter but not in their name. And sending a new value as a signal argument does not add anything useful except requiring QML developers to remember to put an explicit argument to change signal handlers (or else we get a deprecation warning).