Warn on statements that have no effect

[ogoffart]

As discussed in #4942, this Slint code

    FocusScope {
        key-pressed(event) => {
            if (event.text == "a") {
                accept
            }
            reject
        }
    }

Looks like it would return accept, but in fact, the statement has no effect and one should get a warning for that.

And statements with no effect could also be removed, as they may still cause warnings in the generated code.

[ogoffart]

Removing useless statements would hopefully fix the test

slint/tests/cases/issues/issue_4942_no_else_value.slint

Lines 20 to 27 in 028d075

	FIXME: The C++ test currently fails with this warning
	(see also https://github.com/slint-ui/slint/issues/4954)
	//ignore: cpp
	(This is the warning, for reference)
	statement has no effect [-Werror=unused-value]
	| return [&]{ [&]() -> void { if (arg_0) { 45; } else { ; }}();return 12; }();
	| ^~