Fix touchpad camera controls

[slimbuck]

This PR updates the trackpad camera controls to finally be usable:

• two finger swipe to orbit
• shift to pan
• ctrl to zoom

Also fix various little issues:

• handle the case when user drops not-files into the application
• test for presence of serviceWorker before attempting to use it
• use alt and cmd key for brush resize to keep consistent camera controls (cc @simonbethke)

Notes:

• unfortunately windows trackpad often limits swipes to one of the two axes, making it less useful than mac for camera control. this can't be fixed according to https://issues.chromium.org/issues/41468861
• tested on:
  • macos: chrome, safari, firefox
  • windows: chrome, firefox
• firefox wheel mouse detection doesn't work properly. use middle mouse click to zoom instead.