New Options UI

The overall look of the new Options UI, mimicking the Chrome options UI, is pretty much ready. There are still a couple of things, mostly related to functionality in the Quicktexts listing view, that are still in development.

We now have two separate views (the Quicktexts listing and the extension Settings) that you can switch between, with an animation. This should also help in the future, if we'll have more options, since they will no longer fill-up the sidebar.

I've done some re-factoring in the CSS code, and used variables for most of the things, so it should be very easy to tweak the layout from now on. 

- [x] Global styles mimicking the Chrome settings UI
- [x] Split Settings in separate view
- [x] Edit Quicktexts
- [x] Delete Quicktexts
- [x] Improve the UI of the `New quicktext` dialog
- [x] Log-in modal layout
- [x] Sign-up modal layout
- [x] Clean-up older code and templates
- [x] Fix issues with the Popup and Dialog templates
- [x] Don't use transition in the action popup
- [x] Fix issues with clicking on a quicktext inside the action popup
- [x] Check issues with links to the Options page working only when the Options tabs is already open
- [x] Tagging functionality

Bugs
- [x] Tags list is not displayed in the popup view.
- [x] Add/Edit/Duplicate/Delete actions work on both Popup and Options, but the UI is not updated after the action is performed.
- [x] 'Save as quicktext' works only when the 'Options' tab is open.
- [x] Add the 'Tab Completion' in the List of Quicktexts
- [x] Make the Tab completion field `optional`