Detect cyclic context references

[JulienCochuyt]

There is currently no attempt to detect cyclic context reference, neither in the Rule Editor nor when matching rules.
Detecting in the Rule Editor is for sure the less performance costly.
If the check turns out not to be too heavy on resources, implementing it also when matching rules would be the safest.

Labeling as both "bug" and "enhancement" as:

• It is a known and deliberate temporary limitation
• It just turned out to occur in real life (report from Alphonse)