Adds scrolling to sidebar
Simplifies button click event
Can switch between inspector and assets tabs

- added variable for target's class list in mouseclick event
listener as that will be used the most
- assets given the class collapsed
- The function toggleTabs() was temporarily removed till we can
see if it's actually useful or not
Prevents default behavior for all buttons

- Default behavior does stuff like scroll the sidebar to the top
hud background color
Adds collapse/expand functionality for collapsible sections and,

- Makes local value for displayBlocks where it is used (until
we can hook in require())
- Creates utilities.js which currently serves to contain functions
that get data from elements, though may do more later. First
function added gets the all the siblings of an element.