ProcessPendingEvents() was never called when using MFC event loop,
meaning that queued events were never processed, so that using
CallAfter() didn't work.
Fix this and also ensure that these events are processed soon enough by
also implementing WakeUpIdle() for such applications, as wxWakeUpIdle()
didn't do anything neither for them.
Reset m_pMainWnd in wxMFCApp::ExitInstance() to avoid crash when
deleting it again in OnMainWindowDestroyed() that could happen if
ExitInstance() was called not because the main window was closed (normal
case) but because wxApp::ExitMainLoop() was called, as it happens when
an unhandled exception is thrown.
When using two-step creation, as when loading from resources, for
example, it can be convenient to create wxMFCWnd as part of the (parent)
wxWindow object, but only attach it to the real HWND later, once it
becomes available.
This avoids using m_pMainWnd after its HWND becomes invalid, as this
resulted in assert failures from CWnd::WalkPreTranslateTree() called
with this HWND as its hWndStop argument from PreTranslateMessage() which
was used to pre-translate a WM_NULL message the application sometimes
received while closing down.
Still use the active event loop if there is one, just in case it
customizes messages pre-processing, but fall back on the standard
pre-processing code even if there is no active wx event loop and we're
only running the MFC one, as without doing this there are just too many
things that don't work (e.g. menu accelerators didn't work at all in
mixed wx/MFC applications previously).
When porting an existing MFC codebase to wxWidgets, it may be useful to
continue using the existing CWinApp-derived application class, so allow
deriving wxMFCApp from it instead of always deriving it from CWinApp
itself.
Allow reusing this functionality from outside the library, it can he
useful if an MFC window needs to be embedded into a wx application (or
vice versa).
Also use a better wxEntryStart() overload as a side effect, this should,
in particular, fix the problem with command line arguments processing in
mixed MFC/wx applications pointed out in a comment in the sample
previously.
