Infinite loops in ExprTK expressions lock the engine

[texodus]

Bug Report

Steps to Reproduce:

1. Create an ExprTK expression with an infinite loop, for (var i := 1; i;) {}. Don't save the expression or add it to the View, just type it in.

Expected Result:

Nothing, while clicking "save" is bad, this expression is valid.

Actual Result:

A miniature black hole is suddenly created without warning.

Environment:

Perspective 2.10.0, all languages.

[timkpaine]

Xref #2104