Draft: [218] CustomImageNode implementation

See #218

This solution is the 1st out of 2 steps in displaying the Symbol inside a Node. The new CustomImageNode is similar to the existing ImageNodeStyle but adds supports for aql expression and displays the result as a image.

With this MR, the new CustomImageNode has to be added manually through Papyrus Studio. It will be added via the code to the Nodes during the second step.

Note: this MR is based on !29, as the widget is required to select a Symbol in the first place. As such, it is also based on the same Sirius-Web contribution and should not be merged before it is integrated into Papyrus-Web