Inline conditions

[s2b]

Now that Fluid supports boolean literals (like {true}), the next step would be to support more advanced conditions not just in ConditionViewHelpers but right in the inline variable syntax.

Example:

<my:viewhelper disabled="{myString == 'test'}" />
<my:viewhelper disabled="{myString == 'test' && byBool}" />

This can probably be implemented with an ExpressionNode, however we need to be careful to not break existing expressions.

Related: #911