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.c

@@ -8,7 +8,6 @@
 #include "nvim/screen.h"
 #include "nvim/ui.h"
 #include "nvim/os_unix.h"
-#include "nvim/term.h"
 #include "nvim/fold.h"
 #include "nvim/diff.h"
 #include "nvim/move.h"
@@ -20,6 +19,9 @@
 # include "mouse.c.generated.h"
 #endif
 
+static linenr_T orig_topline = 0;
+static int orig_topfill = 0;
+
 // Move the cursor to the specified row and column on the screen.
 // Change current window if necessary. Returns an integer with the
 // CURSOR_MOVED bit set if the cursor has moved or unset otherwise.
@@ -492,3 +494,12 @@ int mouse_has(int c)
     }
   return false;
 }
+
+// Set orig_topline.  Used when jumping to another window, so that a double
+// click still works.
+void set_mouse_topline(win_T *wp)
+{
+  orig_topline = wp->w_topline;
+  orig_topfill = wp->w_topfill;
+}
+