Added option `hltrailingws` for highlighting trailing whitespaces
at the end of lines. Note that it behaves in a "smart" way.
It doesn't highlight newly added (transient) trailing whitespaces
that naturally occur while typing text. It would be annoying to
see transient highlighting every time we enter a space at the end
of a line while typing.
So a newly added trailing whitespace starts being highlighting
only after the cursor moves to another line. Thus the highlighting
serves its purpose: it draws our attention to annoying sloppy
forgotten trailing whitespaces.
Modified behavior of CursorUp, CursorDown, CursorPageUp etc:
if softwrap is enabled, cursor moves by visual lines, not logical lines.
TODO: implement it also for Home and End keys: move cursor to the
visual start or end of a line. I haven't implemented it for now, because
I'm not sure what should be the behavior of StartOfTextToggle then
(considering that Home key is bound to StartOfTextToggle by default).
Fixes#1598
These actions reintroduce the behavior of micro where the Home key
toggles between the start of text (first) and the start of the line.
The same applies for the variant with selection. This commit also
sets these bindings as the defaults.
