clickable and draggable scrollbar (#2431)
clickable and draggable scrollbar (#2431)

* can click to navigate

This deals with #251

* added drag event

* replaced click and drag events with document node events

this allows the mouse to be moved and released off the scrollbar

* now using absolute cursor position in the window

`.offsetY` was measuring from the top of any clickable element, so in the statusbar or on the tabline, it would start measuring from a different point, causing the cursor to jump.

This now uses the absolute y position of the cursor.

* cosmetic changes: use state and class methods

* added private modifiers

* moved private method to satisfy tslint

* changed to public modifiers

* updated to use arrow functions instead of .bind methods

* removed commented out code and fixed linting

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