Update FromString to allow for raw characters

[ChernayaKoshka]

This pull request allows key combinations / sequences to be defined using raw characters, not just C# Enum names. An example of the change:

Combination.FromString(@"A+Oemtilde") can now be Combination.FromString(@"A+`")

In addition, the conversions have been made case-insensitive, so Combination.FromString(@"A+Oemtilde") can now be Combination.FromString(@"a+OeMtIlDe")

Currently, something like Combination.FromString(@"A++") is not supported, however. The + will still need to be OemPlus.

Finally, the C# version has been updated to be C# 7.0.

Please let me know if you have any questions / suggestions.