Open options in a new tab.

Previously, clicking on "options" or "about" link in the question GUI
would open the relevant page in the same window and affect user's
navigation. Now they open in a new tab with target="_blank".
On preferences update, notify all option pages.

If the user has multiple "options" pages opened, modifications in
one of them will now be reflected in others.
Fix flickering iframe by pre-setting border width.
Improve options handling.

- Options now listen to updateUserPreferences event;
- Improve navigation between faux-tab contents;
- Override unnecessary transition when opening options with "#about" hash;
- Some refactoring.
Merge branch 'better-options' into develop

Improves options handling and GUI, fixes bugs.