patch 8.0.1309: cannot use 'balloonexpr' in a terminal

Problem:    Cannot use 'balloonexpr' in a terminal.
Solution:   Add 'balloonevalterm' and add code to handle mouse movements in a
            terminal. Initial implementation for Unix with GUI.
diff --git a/src/term.c b/src/term.c
index 83b54e1..359c171 100644
--- a/src/term.c
+++ b/src/term.c
@@ -5625,6 +5625,7 @@
 		    modifiers |= MOD_MASK_ALT;
 		key_name[1] = (wheel_code & 1)
 					? (int)KE_MOUSEUP : (int)KE_MOUSEDOWN;
+		held_button = MOUSE_RELEASE;
 	    }
 	    else
 		key_name[1] = get_pseudo_mouse_code(current_button,