Small refactoring in wxFileSystemWatcher MSW implementation.

Make wxIOCPService::GetStatus() smarter about its return value, it makes sense
to encapsulate the convention used to indicate the thread exit condition
inside wxIOCPService class itself instead of sharing it between it
wxIOCPThread itself.

It will also make it easier to detect more detailed error conditions in this
code.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/branches/WX_3_0_BRANCH@76180 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775
Fix handling deletion of watched directory in MSW wxFileSystemWatcher.

Don't log an incomprehensible error when the watched directory itself is
deleted, but generate wxFSW_EVENT_DELETE for it. This is consistent with the
behaviour under Unix and generally more useful.

Closes #13294.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/branches/WX_3_0_BRANCH@76181 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775
Fix wxFileSystemWatcher::RemoveAll() to actually work.

We need to call DoRemove() on all watcher objects to really remove them, just
removing our record of them was not enough and e.g. resulted in errors if we
tried to re-add a previously watched path again.

Closes #15531.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/branches/WX_3_0_BRANCH@76182 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775