Add standard design for layertree

[olivierSemet]

Following the spec #1708

• Adding the desktop layertree design
• Splitting layertree.less in 3 files in order to have a different design for mobile/desktop with a common base style
• Refactoring selectors for the future needs (e.g for Defninir l'ordre #819 )

[olivierSemet]

ping @pgiraud

[pgiraud]

Comments about the visual aspect first.

• There is extra space above the first group.
  
• The scrollbar is too close to the groups. This is not pretty.

• Can you make it so the collapse/expand icons are less visible (lighter gray)? The group name is more important than the chevron.
  vs.
• Can you give a try at using smaller size for child nodes? The goal is to emphasize the parent/child relationship. Not sure it's useful, but it's worth trying.

vs.

[pgiraud]

Any reason why you use 2 different shadows?

What about using the following?

box-shadow: 0 0 4px 1px @input-border-focus;

[pgiraud]

Please add a space before curly brace (for consistency).

[pgiraud]

Looks good to me. Please merge.