Added getStyleRange to StyleSpans

[Jugen]

Resolves #1231 How to get real size of one span at position X

With this PR a developer can now get the range of a style that crosses paragraphs.
Both the requested position and the returned IndexRange are relative to the start of the requested StyleSpans.

Usage Example:

var ss = codeArea.getStyleSpans( 0, codeArea.getLength() );  // From beginning to end
var range = ss.getStyleRange( position );  // So range is from beginning