refactor: Remove term modules and termcap options

- Removed term.c, term.h and term_defs.h
- Tests for T_* values were removed. screen.c was simplified as a
  consequence(the best strategy for drawing is implemented in the UI layer)
- Redraw functions now call ui.c functions directly. Updates are flushed with
  `ui_flush()`
- Removed all termcap options(they now return empty strings for compatibility)
- &term/&ttybuiltin options return a constant value(nvim)
- &t_Co is still available, but it mirrors t_colors directly
- Remove cursor tracking from screen.c and the `screen_start` function. Now the
  UI is expected to maintain cursor state across any call, and reset it when
  resized.
- Remove unused code

src/nvim/mouse.h

@@ -25,6 +25,16 @@
 #define MOUSE_MAY_STOP_VIS     0x10    // may stop Visual mode
 #define MOUSE_RELEASED         0x20    // button was released
 
+// Codes for mouse button events in lower three bits:
+#define MOUSE_LEFT     0x00
+#define MOUSE_MIDDLE   0x01
+#define MOUSE_RIGHT    0x02
+#define MOUSE_RELEASE  0x03
+
+#define MOUSE_X1       0x300  // Mouse-button X1 (6th)
+#define MOUSE_X2       0x400  // Mouse-button X2
+
+
 #ifdef INCLUDE_GENERATED_DECLARATIONS
 # include "mouse.h.generated.h"
 #endif