BufView returns not only the buffer's width and height but also its
x,y position. It may be useful e.g. for checking if a mouse click was
on the actual buffer or ourside it, e.g. on the gutter.
Fixes issue with the usage of a slightly incorrect buffer height value
(v.Height should be v.Height-1 if statusline is displayed).
Also, to avoid too many duplications, the code reorganized a little:
buffer display params (width, height, gutter offset and others) are
calculated in a single place.
Softwrap implementation enhanced to fix various issues with scrolling,
centering, relocating etc.
The main idea is simple: work not with simple line numbers but
with (Line, Row) pairs, where Line is a line number in the buffer
and Row is a visual line (a row) number within this line.
The logic remains mostly the same, but simple arithmetic operations
on line numbers are replaced with corresponding operations on
(Line, Row) pairs.
Fixes#632, #1657
