Do not collapse single frames under GroupedStackFrame

[gzsombor]

What it does

Adjust the stack frame grouping logic, so if only one frame is in a group, we don't want to hide it.

How to test

Start debugging, put breakpoint into a function called through a lambda, like:

	void a5() {
		Optional.of("aa").map(this::a6);
	}

	String a6(String s) {
		return s;
	}

Author checklist

• I have thoroughly tested my changes
• The change is following the coding conventions
• I have signed the Eclipse Contributor Agreement (ECA)

[iloveeclipse]

Thanks!

[iloveeclipse]

Fixes this comment: #583 (comment)