Handle Wintab cursors that are recognized after GDK app init

Merged Steven Calwas requested to merge calwas/gtk:issue1549 into gtk-3-24

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 (closed)

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