[Improvements] Allow "static" operators

[mrvux]

PR Pending for that one (still few harmless runtime warnings) :

• Operator should not need a t3 file (since they are pure c#)
• Default value in t3 file replaced by a "Default" custom attribute (to add to Input slot in the instance)
• Mark the symbol as static
• Use a custom attribute to indicate that node is static (not rewritten in the operators csproj).
• Do not write t3 and t3ui file on save for static instances (seems on load there is a small warning if t3ui file is missing, should remove this).
• Operator will not support dynamic compilation in that case, but that's ok.