This shows the original issue on macOS/arm64 (if the rest of the PR is
not applied)
```
GLib.GException: Invalid byte sequence in conversion input
at GLib.Marshaller.FilenamePtrToString(IntPtr ptr) in /Users/poupou/git/external/GtkSharp/Source/Libs/GLibSharp/Marshaller.cs:line 62
at GLib.Marshaller.FilenamePtrToStringGFree(IntPtr ptr) in /Users/poupou/git/external/GtkSharp/Source/Libs/GLibSharp/Marshaller.cs:line 68
at Gtk.FileChooserDialog.get_Filename() in /Users/poupou/git/external/GtkSharp/Source/Libs/GtkSharp/Generated/Gtk/FileChooserDialog.cs:line 222
at Samples.FileChooserDialogDemo.OnPressed() in /Users/poupou/git/external/GtkSharp/Source/Samples/Sections/Dialogs/FileChooserDialogSection.cs:line 28
at Gtk.Button.Pressed_cb(IntPtr inst) in /Users/poupou/git/external/GtkSharp/Source/Libs/GtkSharp/Generated/Gtk/Button.cs:line 388
at GLib.ExceptionManager.RaiseUnhandledException(Exception e, Boolean is_terminal) in /Users/poupou/git/external/GtkSharp/Source/Libs/GLibSharp/ExceptionManager.cs:line 58
at Gtk.Button.Pressed_cb(IntPtr inst) in /Users/poupou/git/external/GtkSharp/Source/Libs/GtkSharp/Generated/Gtk/Button.cs:line 388
at Gtk.Application.Run() in /Users/poupou/git/external/GtkSharp/Source/Libs/GtkSharp/Application.cs:line 115
at Samples.Program.Main(String[] args) in /Users/poupou/git/external/GtkSharp/Source/Samples/Program.cs:line 44
```
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).
Spinner was not spining - should use SetValue, not SetValues.
Also fixed sln - VS2019 not want to open sln by double click in explorer - added empty line.
Also fixed sample project - VS2019 not showing all sources.
