Started work on dialog example
Merge branch 'master' into dialog_example
We have an easier way to check for the last error message.
Dialog resource script for example
Don't leak window handles
Separate view file for dialog example.
Dialog sliders are bound to view model properties
Spaced out sliders better
Added pie graph, which is a custom control with a custom binding.
Better slider value distribution in dialog example
Drawing pie graph in dialog example
Tidied up graph code
Fixed graph bug when blue was at 100%
Fixed link errors in com.h
View models can now have child view models.

A lot of reorganising involved. IUntypedObservable is now treated as a
first class citizen internally. This wasto sort out some dependency
issues. UntypedObservales can now be asked to cast their wrapped value to
a view model reference, which will throw bad_cast if it isn't a child view
model.
GCC doesn't support brace initialisation for references.
More consistency with interface names.
Simpler to specify binding handlers
Sub-dialog bindings are now working.

ValueWrapper<T> now holds a T&
Unused friend declarations