SvgCompose::generateAssembly: Add file extension if needed
SvgCompose::Assemblies: Add save and load functions

With possibility to save assembly by assembly
ViewController: Fix SVG item creation

Add getter in Assembly for project pointer.
Use absolute path to create SVG item using project directory.
Remove CleanLooks
Editor: fix XML preview initialization
Rename MainWindow into CompositorWidget and create a CompositorController

Controller to handle project manipulation
- open/save/close project
- open/close editor (in tabs)
code cleaning