#111 Add _MOTIF_WM_HINTS support

[Donal K. Fellows]

Tk should support setting the _MOTIF_WM_HINTS property
as part of [wm attributes] on Unix/X; IIRC many window
managers (aside from mwm and dtwm/CDE) support this
nowadays as a way of getting client-specified styles
going, and the structure is easy enough to figure out
given a set of Motif header files.

(submitted so the idea doesn't get lost.)

[Joe English]

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See also #220815 for more information on MOTIF_WM_HINTS