Simple performance fix, which is noticeable when editing large files

Instead of walking over each line (which is a red-black tree internally in MD) use the faster helper method GetLineByOffset in Monodevelop (This method implements a binary search). This is called every edit, navigate or hover.
Merge pull request #427 from rojepp/findline_perf_fix

Simple performance fix, which is noticeable when editing large files