qmllint: warn for assignments inside of if conditions

Currently, qmllint does not warn for assignment inside of if conditions, like

if (x = 3) { ... }

It should emit a warning when it finds an assignment inside an if-condition, except if that assignment is inside and extra pair of braces, like:

if ((x = 3)) { ... } 

Note: The QtC code model does warn about this use case.