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.cpp

@@ -30,6 +30,9 @@ Resources2::Resources2()
     this->error = QPixmap(":/error.png");
     this->icon = QPixmap(":/icon.png");
     this->pajaDank = QPixmap(":/pajaDank.png");
+    this->scrolling.downScroll = QPixmap(":/scrolling/downScroll.png");
+    this->scrolling.neutralScroll = QPixmap(":/scrolling/neutralScroll.png");
+    this->scrolling.upScroll = QPixmap(":/scrolling/upScroll.png");
     this->settings.aboutlogo = QPixmap(":/settings/aboutlogo.png");
     this->split.down = QPixmap(":/split/down.png");
     this->split.left = QPixmap(":/split/left.png");
@@ -50,4 +53,4 @@ Resources2::Resources2()
     this->twitch.vip = QPixmap(":/twitch/vip.png");
 }
 
-}  // namespace chatterino
+}  // namespace chatterino