Enhance the inferface to make it clearer

[samuelroland]

When I saw a friend trying the extension today, I realized the interface is not as intuitive as I thought. He was misunderstanding how to create a link (he entered the link and then clicked on the Add button, that is not related to link creation but to link adding to the current collection). The manual is not yet ready so it doesn't help, but it's just a part of the problem. As I wanted to keep the extension popup small, I didn't include titles to zones.
The goal of this issue is to render the interface more obvious on how to do all actions. Some comments are need too.

[samuelroland]

The interface is now much clearer with a title for each zone !

[samuelroland]

A new title "manage links..."

The new interface is ready for the v1.0 release. I leave the issue open to maybe get feedback for future design enhancements...