Fixed bug for clas Studio and class TextBox

[point32]

For class TextBox in project Myra:
Now cursor won't refresh while user is typing
For class Studio in project MyraPad:
Fixed bug that when you Type '<' after a Xml tag such as '<Panel', the code editor will Insert '' before '>' but not after '>'
Before: '<Panel>'
Now: ''
But there are still several bugs in Studio.cs, like: auto indent don't works correctly.
Added class StringExtensions.

[rds1983]

Thanks @IampointZy!
You can do the indentation by using Format Source command(either press Ctrl+F or click Edit/Format Source).