Refactor wxEventLoopSource-related code.

Refactor wxEventLoopSource-related code.

Currently wxEventLoopSource can't be created directly and can only be used to
monitor file descriptors so reduce the API to just wxEventLoop::AddSourceForFD()
and remove AddSource(), RemoveSource() and RemoveAllSources() which couldn't
be implemented for all ports. This makes the code much simpler without any
loss of functionality.

Make wxEventLoopSource responsible for removing itself from the event loop
when it is deleted. This allows to remove IsOk() and Invalidate() methods
making the code simpler and gets rid of various sets/maps which were used
before.

This also allows to support event loop sources in Carbon as well: wxOSX/Carbon
now compiles and works with wxUSE_FSWATCHER==1.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@62475 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775