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

test/functional/ui/screen.lua

@@ -114,7 +114,7 @@ Screen.__index = Screen
 
 local debug_screen
 
-local default_screen_timeout = 2500
+local default_screen_timeout = 3500
 if os.getenv('VALGRIND') then
   default_screen_timeout = default_screen_timeout * 3
 end
@@ -257,6 +257,8 @@ function Screen:_handle_resize(width, height)
     end
     table.insert(rows, cols)
   end
+  self._cursor.row = 1
+  self._cursor.col = 1
   self._rows = rows
   self._width = width
   self._height = height