Issue1549: Recognition of Windows Wintab cursors

When a Windows GDK application that uses a Wintab graphics tablet starts up, any of the tablet's cursors (stylus, eraser, etc.) that have not been used since the OS started will not yet be known to the Wintab driver. During app execution, as the cursors are used, they are recognized by the driver and begin to generate Windows messages.

Previously, GDK queried the Wintab driver for its cursors only during app initialization. The result was that cursors unknown to the driver at that point were subsequently never recognized by GDK.

With this fix, GDK still queries Wintab during app init. In addition, when a WT_CSRCHANGE (WINTAB Cursor Change) message is received and the relevant cursor is not recognized as a GDK Wintab device, Wintab is queried again to update the GDK Wintab devices with the new cursor(s).