Mobile browser (touch UI) issues

[lukehutch]

PhosphorJS doesn't work well on mobile. Specifically:

• Drag events don't work (dragging tabs, draging/droping items, and dragging sliders).
• You also can't drag on a menu to live-select a menu item on touch-up, the way you can with the left mouse button. This means that if you tap on a menu item on the menu bar, which opens a menu, then you try tapping on another menu item in the menu bar with the first menu still opened, it doesn't open the second menu as expected, it simply closes the first menu.
• The UI layout is not reactive. It would be nice if all panes in a layout would collapse into a vertical full-width layout on mobile (at least in portrait mode, or for small phones).
• You can zoom in but not out, and you can't drag the whole page, which can mean that UI elements that are not in a scrollable content pane may not even be visible on the screen or reachable.

There may be other things I haven't noticed. Long-hold does work as a means of simulating right-click, which is nice.