Support quicker timeline panning

[HadrienG2]

Unless I've missed something, it seems the only way to pan the timeline currently is via click and drag. This is not a super nice gesture, it can cause wrist fatigue quite quickly.

Consider supporting some of the following alternatives instead:

• Browser horizontal scrollbar (not currently displayed, maybe some CSS tweaks could enable it without hiding the headers, using some variation of the table display tricks ?)
• Horizontal mouse wheel movement (for mouses that support it, mine does but in aw-timeline it causes zoom in/out instead of horizontal panning for some weird reason)
• Using some modifier key (Shift, Alt...) to switch mouse wheel rotation function between zooming and panning
• Left/right keyboard arrows (fine motion) and maybe also PgUp/PgDown (coarse motion)