Viewport clip/drag for mobile/touchscreen devices.

Viewport clip/drag for mobile/touchscreen devices.

API changes (forward compatible):

- Display: add 'viewport' conf option to turn on and off viewport
  mode.
- RFB: add 'viewportDrag' option to enable/disable viewport dragging
  mode.

Other:

- Add clip mode setting to default UI. For touch devices, clipping is
  forced on.
- Use CSS media queries to adjust visual elements based on screen
  size. Especially disconnected logo size/position and button text size.
- Catch page unload while connected and give a confirm dialog.
- Change mouse button selector to a single button that changes between
  ' ', 'L', 'M', 'R' when clicked (empty means mouse is just being
  moved and doesn't send clicks).
- include/ui.js:setViewClip() routine sets the clipping of the
  viewport to the current size of the viewport area (if clipping is
  enabled).
- include/ui.js:setViewDrag() toggles/enables/disables viewport
  dragging mode.
- Add several images for the UI and for Apple devices:
    - images/clipboard.png: clipboard menu icon
    - images/connect.png: connect menu icon
    - images/disconnect.png: disconnect button icon
    - images/keyboard.png: show keyboard button
    - images/move.png: viewport drag/move toggle button
    - images/settings.png: settings menu icon
    - images/screen_320x460.png: iOS app/desktop link start image
    - images/screen_57x57.png: iOS app icon
    - images/screen_700x700.png: full size noVNC image