Previously, a GDK application handled the Wintab cursors (stylus,
eraser, etc.) only during app initialization. If new cursors were
recognized by Wintab during app execution, the app would not know
about them.
This fix still handles Wintab cursors known during app initialization.
In addition, when Wintab recognizes new cursors and notifies the app
via a WT_CSRCHANGE message, the app handles the new cursors, creating
new Wintab device objects for them.
Closes#1549