* 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
