More flexible solution character handling.

[jpd236]

• Permit "*" as a valid grid character. While Across Lite doesn't support it, other formats do. It can't be entered directly (since it is bound as the shortcut for circling/uncircling a letter to match Across Lite), but it can be entered as a rebus.

• When setting square text or solutions, never set the empty string, even if ToGrid returns one (due to an invalid character, namely a black square (".") or the symbol indicator ("[")). This ensures that bounds checks pass in HasSolutionRebus.

See #185

[mrichards42]

I'm a little less sure of this one since text stuff can be pretty fiddly, but logically it makes sense that we'd want to use the Blank value of " " since it's specifically called that, instead of "".