config: add 'hide-when-typing'

When enabled, the mouse cursor is hidden when the user types in the
terminal. It is un-hidden when the user moves the mouse, or when the
window loses keyboard focus.

config.h

@@ -74,6 +74,7 @@ struct config {
     struct {
         enum cursor_style style;
         bool blink;
+        bool hide_when_typing;
         struct {
             uint32_t text;
             uint32_t cursor;