Scroll Using Click-and-Drag with Middle Mouse Button (#1559)

* ChannelView: Rename mouse event related members

This is more in line with the naming of the other members as well as
future members.

* ChannelView: Add ability to scroll with middle mouse button

* Add scrolling resources

* Use custom icons for scroll cursor

* Slightly refactor scrolling logic

* Respect screen scaling when calculating scroll offset

* Nicer scrolling UX

This change allows scrolling to be feel smoother when close to the
starting point.

* Add scrolling with keeping middle mouse pressed

This mimics the behavior of browsers as well.

* Refactor ChannelView::enableScrolling

* Disable drag-scrolling on left or right click

src/autogenerated/ResourcesAutogen.hpp

@@ -1,5 +1,4 @@
 #include <QPixmap>
-
 #include "common/Singleton.hpp"
 
 namespace chatterino {
@@ -39,6 +38,11 @@ public:
     QPixmap error;
     QPixmap icon;
     QPixmap pajaDank;
+    struct {
+        QPixmap downScroll;
+        QPixmap neutralScroll;
+        QPixmap upScroll;
+    } scrolling;
     struct {
         QPixmap aboutlogo;
     } settings;
@@ -65,4 +69,4 @@ public:
     } twitch;
 };
 
-}  // namespace chatterino
+}  // namespace chatterino