Perform initial refactoring work

Things that were once singletons are no longer singletons, but are
instead stored in the "Application" singleton

Some singletons still remain, and some renaming/renamespacing is left

src/widgets/scrollbar.hpp

@@ -88,8 +88,6 @@ private:
 
     pajlada::Signals::NoArgSignal currentValueChanged;
 
-    pajlada::Settings::Setting<bool> &smoothScrollingSetting;
-
     void updateScroll();
 };