general info on QT as it relates to MuseScore plugins?

You can almost access any class of the Qt Framework. (note that it's Qt not QT).
The easiest way to make a dialog or UI is to use Qt Creator. Qt Creator is installed with your version of Qt normally.

1. Open Qt Creator
2. File -> New File or Project -> Qt -> Qt Designer Form
3. Create your form with the graphic interface. You may want to learn more about Layout Management in Qt
4. Save your file as for example my.ui
5. In your script use the following lines
   
   //read the UI file and create a form out of it
   var loader = new QUiLoader(null);
   var file = new QFile(pluginPath + "/my.ui");
   file.open(QIODevice.OpenMode(QIODevice.ReadOnly, QIODevice.Text));
   form = loader.load(file, null);

   //initialize some widget value if necessary
   form.verticalLayoutWidget.myLabel.text = curScore.title;
   form.verticalLayoutWidget.myLabel2.text = curScore.composer;

   //connect signal
   form.searchBtn.clicked.connect(doSomething);
   form.closeBtn.clicked.connect(close);

   //show the form
   form.show();
   

6. Signals and slots are a Qt concept. When a signal is fired (for example with a button is clicked), slots connected to it are executed. Slots can be define in your script to do something when something happens in the interface

The "break" plugin is a good and simple example of this process. Should I put this in the plugin documentation? An idea for a title?