Layout, implement import/export feature

• Add an import/export feature to the new layout manager to take your layouts to another computer, just like you could already do it with shortcut themes. Theoretically you can use kdenlive-layoutrc manually, but this is not very handy.

• First level would be to save/replace kdenlive-layoutrc from/to a custom destination using the UI.

• Second level would be to add the possibility to import and export only some layouts.

Remark
Its probably necessary to introduce a new file type like .kdenlivelayout for this.