Add pan feature

[whatmusicreallyis]

One extreme useful feature would be the ability to pan the playing field, so that 1/1 can freely move in any direction (as opposed to being stuck in the center).

Ideally, panning would be visually controlled from the keyboard view. A button available under the "Back" arrow would disable the playing field and turn it into a pan anchor. Panning will be achieved by dragging the playing area from any location: with one finger on touch-screen devices, and by holding left mouse button while dragging on legacy devices.

Zooming, that is, controlling visually the "Hex Size (pixels)" setting, would be achieved from the same button but by pinching with 2 fingers on touch-screen devices, and by scrolling the mouse wheel while holding left button pressed on the others.

However, in the beginning, a horizontal+vertical double control on the Settings page (or just an url hack!) only for panning would make do. Moving units could be pixels, or hexagon units.

Programming steps:

1. implement panning feature
2. add visual control for pan & zoom