bookmarks: save bookmark metadata as bookmarks are modified and fix several defects

continuation of !31 (closed)

Fixes #55 source mark iterator skipping source marks at first line of buffer.

while looping through iterator missed first nonoccurence as it has already moved to next mark.

fix is to use do-while loop instead while loop

Fixes #56

bookmark set to last line is not restored. total lines calculated from buffer was 0-indexed hence boundary condition was wrong. made minor fix.