Decouple deployment preferences panels from deployment preferences page

[akerekes]

@briandealwis 's comment:

Could you mark somewhere with a TODO to decouple this class from the specific preference panels? One possibility is to use an extension point where we associate a panel class with a facet type:

<extension point="com.google.cloud.tools.deploy.ui.properties">
  <facet id="com.google.cloud.tools.eclipse.appengine.facets.flex" version="1"
    panelClass="com.google.cloud.tools.eclipse.appengine.deploy.ui.FlexDeployPreferencesPanel" />
<extension>