[Feature] Chrome.commands API shortcuts: added different defaults for Macs
[Fix] Multiple fixes

- [Fix] Observe everything possible using the mutation observer. This is because CSS changes might happen when any attribute,
changes esp. "class" (which might result in overlay needing to be resized), and simply observing changes in the "style" attribute
doesn't suffice in this scenario. (In a future commit, the relatively expensive call to update CUs will happen only in response
to specific DOM changes, while we'll handle other less significant DOM changes simply by redrawing overlays.

- [Fix] stop mutation observer at the beginining of showOverlay (and removeOverlay). This is because jQuery's find('*') [line 804
in mod_CUsMgr.js] seems to (temporarily) change ids of some DOM elements, which creates a infinite cycle (after the fix just mentioned
above in which we respond to *all* DOM changes by updating the CUs array.

- [Fix] Replace left over calls to dom's native addEventListener() by our wrapper mod_domEvents.addEventListener() in all content
scripts where they were still present

- [Fix] In mod_selectFocusables and mod_keyboardLib: move the call to mod_domEvents.addEventListener in the setup() so that these are
 called everytime the module is setup() after a reset()

- [Fix] In the main module, wait till dom ready before initializing extension (prevents flickering of the intially selected CU)
Merge branch 'master' of https://github.com/MakeBetter/Units