add spin button examples
[Wpf] Fix issues with PopoverBackend

- De-parent the content when the popover is hidden
- Give the popover a semi-transparent background
 (other backends all have backgrounds- use the same color as Gtk here)
Fix memory leak caused by undisposed memorystream.
Merge pull request #385 from mono/fixMemoryLeak

Fix memory leak caused by undisposed memorystream.
[Gtk3] fix build on windows
 * don't provide package names in project file
[Gtk3] fix opacity support flags for windows

The recent Gtk3 version for Windows is 3.6 and lacks widget opacity/alpha
support. Opacity feature should be disabled, until a newer Gtk3 version
is released for Windows.

Window opacity is supported on Windows and should be enabled.
add SpinButton custom formatting support

adds ValueInput, ValueOutput events for custom
value-text, text-value conversion. Additionally
the shown text can be manipulated through the new
Text property.
[Gtk] add SpinButton custom formatting support
[Mac] add SpinButton custom formatting support
[Wpf] add SpinButton custom formatting support
add custom formatted SpinButton samples
Merge remote-tracking branch 'sevoku/fix-gtk3-build'
Merge branch 'sevoku/extend-spinbutton-formatting'

Conflicts:
	TestApps/Samples/Samples.csproj
	TestApps/Samples/Samples/SpinButtonSample.cs
	Xwt/Xwt.csproj