Updating to work with new tab dragging

onMouseUp on HTML tag now triggers tabDragEnd
Tabs no longer have 'draggable' attribute, onMouseDown replaces onClick,
which also fires tabDragStart event and onMouseUp also fires tabDragEnd
newTab has an ID so we can control it
tab & newTab absolute positioning & zindex of 1

So we can control their position easily
New tab dragging system & del removeSnippet

Previously we were using HTML5 dragging, but this is too freerange, we
want to restrict to a region and x-axis only. Replaced with new system,
doesn't actually do anything yet when you release mouse but UI works.
Deleted the removeSnippet function, no longer used.
Classes to transition tabs left 0.15s

By default this is 0s, tabs other than the one being dragged is given
the extra tabSlide class so they slide around in the background. The tab
being dragged will also slide into place when the mouse button is
released.
Tab sliding and opacity

On start of the drag, all tabs apart from the one we are dragging are
given the additional tabSlide class so they slide in the background.
Better use of swapWith variable and now setting background tabs position
more effectively plus also setting opacity and returning to full opacity
on release of mouse button.
On that event we also add the tabSlide class on our released tab so it
slides into place, set the tab widths/positions again and after 150ms
(after our released tab has finished it's slide transition), reset the
zIndex, classname for everything back to the initial state.