mouse: Implement horizontal scroll. #3450

- Code from Vim source.
- Removed the check for 'guioptions'
- mouse_spec.lua: test <ScrollWheelLeft> and <ScrollWheelRight>
- Move horizontal scroll logic to mouse.c
- Remove 'gui_' from the function names
- Renamed variables to be more specific (as opposed to generic p, w).
- Marked some functions as `static`

src/nvim/mouse.h

@@ -34,6 +34,12 @@
 #define MOUSE_X1       0x300  // Mouse-button X1 (6th)
 #define MOUSE_X2       0x400  // Mouse-button X2
 
+// Direction for nv_mousescroll() and ins_mousescroll()
+#define MSCR_DOWN       0     // DOWN must be FALSE
+#define MSCR_UP         1
+#define MSCR_LEFT       -1
+#define MSCR_RIGHT      -2
+
 
 #ifdef INCLUDE_GENERATED_DECLARATIONS
 # include "mouse.h.generated.h"