GDK W32: More flexible modal operation mode

Instead of using a boolean to indicate a modal operation being in progress,
use a set of flags, and allow these to be set and unset independently.

Specifically, this allows WM_CAPTURECHANGED handler to only act when a drag-move or
drag-resize modal operation is in progress, and ignore DND (which can also cause
WM_CAPTURECHANGED to be posted). This avoids a crash due to assertion failure when
OLE2 DND code tries to end a modal operation that was already ended by the WM_CAPTURECHANGED
handler.

https://bugzilla.gnome.org/show_bug.cgi?id=786121

gdk/win32/gdkglobals-win32.c

@@ -73,7 +73,7 @@ GdkWin32DndState  _dnd_source_state = GDK_WIN32_DND_NONE;
 gint		  _gdk_input_ignore_wintab = FALSE;
 gint		  _gdk_max_colors = 0;
 
-gboolean	  _modal_operation_in_progress = FALSE;
+GdkWin32ModalOpKind	  _modal_operation_in_progress = GDK_WIN32_MODAL_OP_NONE;
 HWND              _modal_move_resize_window = NULL;
 gboolean	  _ignore_destroy_clipboard = FALSE;