Make GenericEditor default editor for org.eclipse.core.runtime.text

Make generic editor default for the base `org.eclipse.core.runtime.text`
content type.

This shouldn't be a breaking change, given that with
b3137d1 customers expecting "old
classic" text editor to be opened by default could customize their
products by adding this line in product customization file:

`org.eclipse.ui.workbench/defaultEditorForContentType_org.eclipse.core.runtime.text=org.eclipse.ui.DefaultTextEditor`

Fixes #2527

Author: iloveeclipse

bundles/org.eclipse.ui.genericeditor/plugin.xml

@@ -32,7 +32,7 @@
       <editor
             class="org.eclipse.ui.internal.genericeditor.ExtensionBasedTextEditor"
             contributorClass="org.eclipse.ui.editors.text.TextEditorActionContributor"
-            default="false"
+            default="true"
             icon="icons/full/obj16/generic_editor.png"
             id="org.eclipse.ui.genericeditor.GenericEditor"
             name="%genericEditor_name">