Only grab scrollwheel buttons if necessary.

With this patch, we only grab the scrollwheel buttons (4 and 5) when
managing a window if a whole window key binding exists for these buttons.

This allows both of these usecases:
  - Bindings to scrollwheel buttons using --whole-window (see #1701).
  - Scrolling in a window without focusing it if no such binding
    exists (see #2049).

Furthermore, we drop all button grabs and regrab them after a config
reload in order to reevaluate the new bindings correctly.

fixes #2049

include/bindings.h

@@ -33,6 +33,13 @@ Binding *configure_binding(const char *bindtype, const char *modifiers, const ch
  */
 void grab_all_keys(xcb_connection_t *conn);
 
+/**
+ * Release the button grabs on all managed windows and regrab them,
+ * reevaluating which buttons need to be grabbed.
+ *
+ */
+void regrab_all_buttons(xcb_connection_t *conn);
+
 /**
  * Returns a pointer to the Binding that matches the given xcb event or NULL if
  * no such binding exists.
@@ -95,3 +102,12 @@ CommandResult *run_binding(Binding *bind, Con *con);
  *
  */
 bool load_keymap(void);
+
+/**
+ * Returns true if the current config has any binding to a scroll wheel button
+ * (4 or 5) which is a whole-window binding.
+ * We need this to figure out whether we should grab all buttons or just 1-3
+ * when managing a window. See #2049.
+ *
+ */
+bool bindings_should_grab_scrollwheel_buttons(void);