Escape a literal # in the iemguis so that it can be used inside labels.

This fixes #427.

We do this by tacking on a tilde if # is followed by a digit, to prevent it
from being mistaken for a $ substitution (which happens because $ becomes # in
FUDI, which is required behavior by non-iem gui objects and thus we don't want
to break this). The tilde gets removed automatically when the label is sent to
the gui.

All this is implemented using the existing symbol mungers in the
iemgui_dollar2raute and iemgui_raute2dollar functions in g_all_guis.c and
iemgui_escape and iemgui_unescape in dialog_iemgui.html.
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