Rework of the file selection widget for representations

[BenSouchet]

Goal / To Do

Improve the widget of the file selection for representations, the major goal is to be able to easily add representations and review items.
The widget need to work for all the family and render types, by using configurable variables to set:

• a bool to enable (or disable) the ability to add thumbnail to representation items
• a list of whitelisted file extensions for the representation items
• a list of whitelisted file extensions for the review items
• The list of extensions allowed for the representations need to be displayed with a QLabel below the treeWidget
• The list of extensions allowed for the reviews need to be displayed with a QLabel below the first label.
• By default representation items without "child" (a.k.a second level item) are also use for review is the file extension is in the list of whitelisted file extensions for the thumbnail items.

In the widget tree:

• top level elements are always representation items
• Second level elements (linked to a top level item) are always review items
• Top level element without second level element(s) have a purple background (meaning representation AND review)
• Top level element WITH second level element(s) have a orange background (meaning representation with custom review(s))
• Second level element have a yellow background
• A "Legend:" QLabel exists to explain the meaning of the colors. It's placed after the two QLabel that store the list of authorized extensions.

Colors / styles can be update later but the fondation should be coded.

• Re-organization of the elements in the widget need to be done via drag n drop
• New items can be added via drag n drop from the computer into the widget
• items (top or second level) can be delete by clicking a trash icon

Create Behavior

• When creating the publish element, the representation(s) need to be instantiated to then be able to publish them.