Fix two digit positions (separated with '-')

[rhh]

The original syntax check was done in ParseChord(), but the regular expression was broken (->fixed). Errors at this point lead to a big error-cross instead of a chord symbol.

Before that, there is a rough length of "positions" and "fingering" test in GenerateChordHtml(). This test is extended (as proposed) to allow more than 6 chars if there is a '-' in "positions".

Test-case is added to "test.html".

I personally don't like the checking to be done at two different places...