Fixes various bugs caused by the QML engine bypassing setters
when BINDABLE is specified (even if the bindable is const).
Also restructures all properties using BINDABLE to have
a default READ and WRITE to ensure this doesn't happen again.
An experiment that should reduce boilerplate for properties that just
access a backing value. Code also exists for using it as an interface
for other properties as well, but isn't currently in use.
