Add stub DOCTYPE declarations for Eclipse plug-in manifest files

We actually know the full grammar for these files: it is documented at
<http://help.eclipse.org/kepler/index.jsp?topic=%2Forg.eclipse.platform.doc.isv%2Freference%2Fmisc%2Fplugin_manifest.html>.
We ought to be able to extract that DTD into a file and give each
"plugin.xml" a "<!DOCTYPE plugin SYSTEM ...>" declaration referencing
it.  Unfortunately, that leads to a new warning: "External entity
resolution is not supported by PDE."  So a stub declaration is the
best we can do.  Fortunately, Eclipse's structured editor seems to
preserve these once we add them by hand.

com.ibm.wala.core.tests/plugin.xml

@@ -1,5 +1,6 @@
 <?xml version="1.0" encoding="UTF-8"?>
 <?eclipse version="3.2"?>
+<!DOCTYPE plugin>
 <plugin>
    <extension id="HeadlessWALA"
          point="org.eclipse.core.runtime.applications">

com.ibm.wala.ide.tests/plugin.xml

@@ -1,5 +1,6 @@
 <?xml version="1.0" encoding="UTF-8"?>
 <?eclipse version="3.2"?>
+<!DOCTYPE plugin>
 <plugin>
    <extension id="HeadlessWALA"
          point="org.eclipse.core.runtime.applications">