Guard against CharView UI Hints/View menu crashes

This change implements the strategy "disallow user actions when they
are not appropriate - disable inappropriate UI menu items".  Thus we
add code that checks whether the current charview display layer is
'Grid' and disables menu items that are known to crash when 'Grid'
is selected.

This change adds disabling of the following charview menu items when
the 'Grid' layer is selected:

    Hints menu
        Hint Substitution Pts
        Clear HStem
        Clear VStem
        Clear DStem
        Create HHint...
        Create VHint...
    View / Number Points menu  (all items)
        None
        TrueType
        PostScript
        SVG
        Position

This is not necessarily an exhaustive list of all vulnerable UI actions,
just those which were proven to crash either always or 'sometimes'.

Tested as not affecting the menu items when 'Back' or 'Fore' layers are
selected.

Note that this change does **not** correct the vulnerable internal
routines.  (*That will be another change*)  This change merely prevents
those vulnerable routines from being reached by user menu actions known
to be 'bad'.

The patient says, "Doctor, it hurts when I do this." "Then don't do that!"
Merge pull request #1754 from tshinnic/1750_invalid_layer_index_1

Guard against CharView UI Hints/View menu crashes