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.

src/nvim/globals.h

@@ -918,10 +918,6 @@ EXTERN disptick_T display_tick INIT(= 0);
  * cursor position in Insert mode. */
 EXTERN linenr_T spell_redraw_lnum INIT(= 0);
 
-/* Set when the cursor line needs to be redrawn. */
-EXTERN int need_cursor_line_redraw INIT(= FALSE);
-
-
 #ifdef USE_MCH_ERRMSG
 // Grow array to collect error messages in until they can be displayed.
 EXTERN garray_T error_ga INIT(= GA_EMPTY_INIT_VALUE);