Merges everything from the branch with only some minor changes, mostly renamed
wxUSE_FSWATCHER_{INOTIFY,KQUEUE} to wxHAS_{INOTIFY,KQUEUE}.
Add wxFileSystemWatcher and related classes.
Also introduces wxEventLoopSource.
git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@62474 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775