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/widgets/helper/ChannelView.hpp

@@ -148,6 +148,9 @@ private:
     void updatePauses();
     void unpaused();
 
+    void enableScrolling(const QPointF &scrollStart);
+    void disableScrolling();
+
     QTimer *layoutCooldown_;
     bool layoutQueued_;
 
@@ -183,15 +186,26 @@ private:
     bool onlyUpdateEmotes_ = false;
 
     // Mouse event variables
-    bool isMouseDown_ = false;
+    bool isLeftMouseDown_ = false;
     bool isRightMouseDown_ = false;
     bool isDoubleClick_ = false;
     DoubleClickSelection doubleClickSelection_;
-    QPointF lastPressPosition_;
+    QPointF lastLeftPressPosition_;
     QPointF lastRightPressPosition_;
     QPointF lastDClickPosition_;
     QTimer *clickTimer_;
 
+    bool isScrolling_ = false;
+    QPointF lastMiddlePressPosition_;
+    QPointF currentMousePosition_;
+    QTimer scrollTimer_;
+
+    struct {
+        QCursor neutral;
+        QCursor up;
+        QCursor down;
+    } cursors_;
+
     Selection selection_;
     bool selecting_ = false;
 
@@ -211,6 +225,8 @@ private slots:
         queueLayout();
         update();
     }
+
+    void scrollUpdateRequested();
 };
 
 }  // namespace chatterino