Implement expand tab functionality, make tabwidth configurable

If expandtab is enabled then inserted tabs are replaced by tabwidth
amount of spaces.

Both settings apply to all windows files and can be changed via:

 :set tabwidth n                          # where 1 <= n <= 8
 :set expandtab (1|yes|true)|(0|no|false)

1 files changed, 2 insertions, 0 deletions

diff --git a/window.h b/window.h
index 46322aa..ec52f48 100644
--- a/window.h
+++ b/window.h
@@ -24,6 +24,8 @@ void window_move(Win*, int x, int y);
 void window_draw(Win*);
 /* flush all changes made to the ncurses windows to the screen */
 void window_update(Win*);
+/* changes how many spaces are used for one tab (must be >0), redraws the window */
+void window_tabwidth_set(Win*, int tabwidth);
 
 /* cursor movements which also update selection if one is active.
  * they return new cursor postion */