Bug 37197 Basepath for relative pathes in JAR-exporter
The basepath for the JAR-exporter is the eclipse-install-directory. I prefer it to be the root of the project (the description is saved in), so that relative pathes end inside the project. Now: <jardesc> <jar path="workspace/projectdir/jar.jar"/> Desired: <jardesc> <jar path="jar.jar"/> Reason: To make the jardesc-files portable for shared projects
org.eclipse.jdt.ui/ui/org/eclipse/jdt/ui/jarpackager/JarPackageData.java
* 
	 * @return <code>true</code> if the JAR Package info is valid
	 */
	public boolean isValid() {
		return (areClassFilesExported() || areJavaFilesExported())
			&& getElements() != null && getElements().length > 0
			&& getJarLocation() != null
			&& isManifestAccessible()
			&& isMainClassValid(new BusyIndicatorRunnableContext());
	}
	/**
	 * Tells whether a manifest is available.
	 * 
	 * @return <code>true</code> if the manifest is generated or the provided one is accessible
	 */
	public boolean isManifestAccessible() {
		if (isManifestGenerated())
			return true;
		IFile file= getManifestFile();
		return file != null && file.isAccessible();
	}
	/**
	 * Tells whether the specified manifest main class is valid.
	 * 
	 * @return <code>true</code> if a main class is specified and valid
	 */
	public boolean isMainClassValid(IRunnableContext context) {
		return JarPackagerUtil.isMainClassValid(this, context);
	}
}