Support staff lines without a starting bar line

[sonovice]

Right now the vanishing point computation (and some other things) depend on staff lines starting and ending with a bar line. Especially single instrument staves usually do not have such a starting line, resulting in various errors.

Basic idea:

1. Do staff line detection as already implemented
2. Draw blobs at the start and end of detected staff lines
3. Use Hough Transform on these "blob images" do detect the "non-existent bar lines"
4. Profit.