Touch event support

[jjrv]

The old phosphorjs/phosphor#154 issue remains unresolved. Touchscreen laptops and larger tablets, both with touch events, could support a desktop-style web app. However Lumino DockPanel currently only responds to mouse events.

Another UX issue is that even with touch event support, the spacing between panels is a very small area to hit for resizing them. A larger handle, gesture and / or resizing mode is needed to manage them comfortably with a finger as the only pointing device.

I propose two options:

• A resizing mode that app developers can decide how to trigger and render, but would add a floating resize widget in the middle of every panel subdivision, large enough to drag.
• A gesture: dragging with two fingers, each in a different panel, could move the divider between the panels.

[blink1073]

It looks like pointer-events are fully supported by the browsers we are targeting (recent Firefox, Chrome, Edge, and Safari) and many of the touch-action events are supported.

https://developer.mozilla.org/en-US/docs/Web/CSS/pointer-events
https://developer.mozilla.org/en-US/docs/Web/CSS/touch-action

[blink1073]

cc @tgeorgeux for UX input on the gesture and resizing