Fixed glib source double removal then using Source.Remove (Timeout.Remove, Idle.Remove).
I don't know if fix is correct or safe, but it solves problem that you can test in TimerDemo section from samples.
Repro:
If on Windows you should enable console in samples: <OutputType>Exe</OutputType>
Go to TimerDemo section and press buttons:
1. Add timer
2. Remove timer by handler
3. GC - no error in console
1. Add timer
2. Remove timer
3. GC - error in console "GLib-CRITICAL **: 20:29:41.579: Source ID 123 was not found when attempting to remove it"
* Fixed marshaling for reading child properties.
* Enabled child properties for Box, Grid, Stack in api xml.
* Added sample/test for using child properties (ChildPropertiesSection).
