GUAC-955: Fix handling of scroll on iDevices

The various iOS devices magically scroll the browser viewport when the on-screen keyboard is shown, instead of simply resizing it. This change adds a guacViewport directive which maintains its position on the screen, and automatically resizes itself to fit always within the viewable area, such that this scroll effect is nullified.

As "overscroll" is the default browser action when a scrollable region (such as the menu) is dragged past its scrolling limits, this change also removes browser control of menu scroll, instead implementing that scrolling manually.