[Refactor/Broken] Remove code for allowing space as a modifier, fixes #145

Notes:
1) In particular we are removing the ability to specify
 space as a regular modifier in calls to mod_keyboardLib.bind(). However, we will continue to allow space to be used as a "special" modifier, for example by mod_selectLink. (and perhaps elsewhere in the future). Hence, some of "space-as-modifier" code continues to remain in mod_keyboardLib, and some more will be added in the subsequent commit
 2) This commit will partially break select-any-link; this will be fixed in the next commit
[Refactor] Move code related to "space as a *special* modifier" from mod_selectAnylink to mod_keyboardLib

- Will enable space to be used as a modifier with non-character keys (like arrow keys) by other modules if required
 - Will enable space to be as a shortcut even when not used as a modifier (on keyup etc) by another module (will be done in an upcoming commit)
[Feature/Refactor] Allow mod_smoothScroll to scroll elements horizontally by accepting additional argument `scrollProperty` which can be passed either 'scrollTop' or 'scrollLeft'

- Change existing calls to specify 'scrollTop' for this argument
[Feature/Broken] Implemented horizontal scrolling ('h' and 'l' by default)

Also, added data for horizontal CU navigation in defaultSettings.js (and changed terms like 'nextCU', 'prevCU', etc'  to words with 'up', 'down', now that there is going to be 2D navigation

 This will break existing mod_CUsMgr functionality, which will be fixed in the next commit
[Feature] Implement two dimensional movement in CU selection (now supports all four directions)
[Feature] scrollCUIntoView() modified to handle horizontal movements between selected CUs
Plus, misc minor related changes
[Minor] Updated defaultSettings.html based on latest defaultSettings.js
[Fix] Update  property name to correspond with recent changes
 TODO: helpUI.js needs to be fixed as well