[Refactor] Simplify code
[Fix] Remove erroneous check
[Fix] Fix for case when destination scroll position is the same as staring scroll position
[Fix] Minor fix (since case `e.which` works anyway)
[Feature/Usability] Allow the keys Space and "thumb modifier" to scroll up/down (by CUs if the page has those, else normally)

- thumb modifier: cmd on Mac, alt elsewhere
[Fix] Fixes 2 minor issues
- sometimes first CU wasn't getting selected on page load
- CU wasn't getting selected automatically on reenabling extn after temporarily disabling it
[Fix] fix regarding use of space key as modifier
related fix in mod_selectLink: pressing down space by itself won't remove focus from the active element
[Minor] change keycode to keyCode
[Refactor] Move keydown handler before keyup handler + add comments
[Feature] Power user feature: mod_easyScrollKeys: allow quick repeated scroll/select CU if space/thumb-modifier are held down after a double tap (since it normally works only on keyup)
[Fix] fix the issue of inadvertent scroll-ups ups on the thumb-modifier key's keyup, when thumb modifier is involved in a keyboard shortcut (like cmd + <key>) to change tabs
[Data] added some url-data
[CSS] Minor changes