Added ability to cut and copy the current line in UITextInput without selecting

By current line, I mean the line with the caret on it (or, in PolyUITextInput.cpp, `lineOffset`). You just do the normal keyboard shortcut for either of those with nothing selected, and it copies/cuts the line to the clipboard.

I think I should note that `UITextInput::removeLines()` seemed to be acting funny when I tried to use it on single-line UITextInputs. I ended up just clearing out the text on those on cut with a `setText("")`.