Workspaces: fix twitchy scrolling on touchpads

Previously, any and all scroll events were interpreted as reason to switch
workspaces. This resulted in twitchy behaviour, where the scrolling was
practically unusable.

Now, we pool all scroll values, and only scroll if the value is larger than the
new config option "smooth-scrolling-threshold". If this option is not set, the
behaviour is unchanged.

include/modules/sway/workspaces.hpp

@@ -41,6 +41,7 @@ class Workspaces : public IModule, public sigc::trackable {
   util::JsonParser                             parser_;
   bool                                         scrolling_;
   std::unordered_map<std::string, Gtk::Button> buttons_;
+  gdouble                                      distance_scrolled_;
 
   util::SleeperThread thread_;
   Ipc                 ipc_;