513306: Propose solutions for missing natures

Cleanup: with both file type and nature discovery in now, it makes sense
to have a dedicated discovery package.

Also, cleaned up/unified naming a bit and extracted several inner 
classes

Bug: 513306
Task-Url: https://bugs.eclipse.org/bugs/show_bug.cgi?id=513306

org.eclipse.epp.mpc.ui/plugin.xml

@@ -66,13 +66,13 @@
          point="org.eclipse.ui.startup">
       <startup class="org.eclipse.epp.internal.mpc.ui.wizards.MarketplaceDropAdapter"/>
       <startup
-            class="org.eclipse.epp.internal.mpc.ui.MissingNatureDetector">
+            class="org.eclipse.epp.internal.mpc.ui.discovery.MissingNatureDetector">
       </startup>
    </extension>
       <extension
          point="org.eclipse.ui.ide.unassociatedEditorStrategy">
       <strategy
-            class="org.eclipse.epp.internal.mpc.ui.AskMarketPlaceForFileSupportStrategy"
+            class="org.eclipse.epp.internal.mpc.ui.discovery.AskMarketplaceForFileSupportStrategy"
             id="org.eclipse.epp.internal.mpc.ui.askMarketPlace"
             label="%unassociatedStrategy.label"
             interactive="true">