display: unify cursorline and concealcursor redraw logic

There is various places where 'conceallevel' and 'concealcursor'
necessitates additional redraws. This tries to separate the different
cases and handle each accordingly:

  - Share code with 'cursorline' for the common case: vertical move of
    cursor within the same window (concealcursor not active)
  - Improve the logic for managing 'concealcursor' and switching modes:
    test for the case where the new mode behaves differently from the
    last one.
  - Clarify the special case for horizontal movement within a line when
    'concealcursor' is active, now there is an if-statement only for this
     and not hidden in larger check mostly for the first point.
  - Keep the special case for moving between windows as is.

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