uses a map or set of URLs, which can be a performance hog #625

reported by a sonarqube analysis, several classes & methods are affected:

• org.locationtech.udig.catalog.ui.FileConnectionPage.openFileDialog(Composite)
• org.locationtech.udig.catalog.ui.FileConnectionPage.checkDND(FileDialog)
• org.locationtech.udig.catalog.ui.FileConnectionPage.list
• org.locationtech.udig.catalog.memory.MemoryServiceExtensionImpl.createService(URL, Map)
• org.locationtech.udig.catalog.memory.MemoryServiceExtensionImpl.impl