Navigationfix when closing addons

[a1rwulf]

This adds a new message to trigger PREVIOUS_WINDOW from a python addon.
When calling "close" from an addon we now trigger the new message instead of ACTIVATE_WINDOW.

Motivation and Context

Scenario:

• You started up kodi and went into the tvshow section.
• You have access to an addon there and open it.
• The addon is not a dialog window but a normal window.
• When you hit back to close the addon, you are in the tvshow section again.
• When you hit back again, you assume to be taken to the home screen again, instead
  you are in the folder view of the MyVideoNav.xml

This happens because the ACTIVATE_WINDOW initializes the window without a start directory.
I can not think about any case where you want to not go "back" when closing an addon.

Maybe anyone can judge if this change is correct!?

How Has This Been Tested?

Tested with kodi master and a windowed addon.

Types of change

• [ x ] Bug fix (non-breaking change which fixes an issue)
• Improvement (non-breaking change which improves existing functionality)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

@Paxxi Do I need any more documentation when I add new Appmessenger messages?
@xhaggi You've been in the reviewer suggestions ;)

[xhaggi]

IMO looks good and makes totally sense because addon windows behaves different and Window::close() should be the same as CGUIWindow::OnBack().

bool CGUIWindow::OnBack(int actionID)
{
  g_windowManager.PreviousWindow();
  return true;
}

[a1rwulf]

Cool.
Just in case anyone wonders:
If your addon is a Dialog, close is handled by WindowDialogMixin.cpp which sends some other special message that ends up to be a OnBack later on, see:
https://github.com/xbmc/xbmc/blob/master/xbmc/interfaces/legacy/WindowDialogMixin.cpp#L45