Monitor UI needs work on touch

[paulkaplan]

/cc @carljbowman

• Double tapping to change monitor styles doesn't work on iPad.
• The sliders do not seem to work on iPad, and may also be too small.
• Dragging monitors drags the whole page around as well.
• The "+" is too small for touch
• The resize handler for list monitors doesn't respond to touch

[picklesrus]

A more specific, but related issue with list monitors on touch: scratchfoundation/scratch-vm#1155

[picklesrus]

And another one: #2158