Track down all manual calls to dispose() in the IntelliJ Plugin

[jacob314]

Some classes like FlutterWidgetPerf
implement the Disposable interface but have dispose()
called manually which is counter to how dispose() should be called for a properly functioning IntelliJ plugin.

Mixing manual and automatic calls to dispose leads to confusing bugs like
#6042

[stevemessick]

FlutterWidgetPerf.dispose() is called by FlutterWidgetPerfManager.dispose(), which is itself never called due to using the project as a dispose parent: Disposer.register(project, this);

We need to fix #6076 before working on this one.