New update to sidebar toggle styling

Update to UI/UX and language file naming conventions.

This is an improvement proposed to the collapsible sidebar introduced since 3.3.7-dev by @roland-d.
It is based on UI proposal by @dbhurley : https://github.com/joomla/joomla-cms/pull/4450
And on ideas from many contributors! :+1: 

In this PR, a sliding effect is added to the sidebar to improve UX, and UI was changed to a column styling.
The sidebar is LTR/RTL ready and is displayed on screen with a min-width of 768px.
Screens under 767px wide use the sidebar as it is already displayed in 3.3.6.
Future improvement with mobile devices could be proposed in another PR.

Intructions for testing : 
- tests on all browsers are welcome
- test to be done in LTR and RTL
- tests with third party extensions are needed

Comments and tips are welcomed!

Cyril

![j-toggle-sidebar-2](https://cloud.githubusercontent.com/assets/2385058/5111900/0a4e9664-7024-11e4-8846-ab52f8573d6a.jpg)

We can have color: @linkColor; here to make the button more visible