Current implementation always says that first 5 lines of code is shown
when selected code does not fully fit into its window. This is not
correct as the window can be resized, may have a header occupying one
line, and the lines may not be the first ones. Also the wording is
awkward.
Correct these issues by checking current window height, accounting for
the potential header presence by checking sidebar.enabled config flag,
and updating the message format to be more concise:
" Selected Code (4/15 lines)"
While at it switch to using Utils.count_lines() which does not need
creating temporary tables.
Note that this does not deal with the header info being stale after
resizing sidebar windows.
c6d2783975