[Refactor/Fix] Move code related to settings etc to background scripts.

- Other related chanoges
- getImmutableCopy() replaced with makeImmutable() which is simpler
and handles arrays correctly as well
[Refactor] Create mod_keyboardLib.js out of mousetrapExtensions.js

- And related changes
[Fix] Fix code in scrollSelectedCUIfRequired to correctly calculate the CU's dimensions
[Data] Add std_header selector for quora
[Fix] Fix position of scrolling marker on scroll down + show on scroll up as well

- For scroll down, the header height was not correctly incorporated earlier. Now fixed.