QML: fix warning in tests

Mark unused fields as Q_UNUSED. They are still "used" to check if the
compiler will generate warnings/errors (which it shouldn't).

Change-Id: I38e1668e88d94ab8f26a92a06f5207b77369e136
Reviewed-by: Simon Hausmann <simon.hausmann@digia.com>
QML Debugging: Fix crash when stepping through try-catch block.

Also fix the stack-trace generation, otherwise the debugger engine would
report a breakpoint hit on the wrong line.

Task-number: QTBUG-42723
Change-Id: I1f655a5174b28a1c9c31c85bbe023fbce5ddbb96
Reviewed-by: Simon Hausmann <simon.hausmann@digia.com>
Windows: Don't disable JIT

[ChangeLog][QtQml][Windows] V4 JIT was re-enabled after it was disabled by mistake in 5.4.0.

Task-number: QTBUG-43171
Change-Id: I16206e094c210b1eadbfef9ee04cf78d3a7dcebe
Reviewed-by: Simon Hausmann <simon.hausmann@digia.com>