Improve validation of XML manifests

[dryabov]

Fixes issue #202 and adds few more checks for XML manifests:

• check that plugin manifest contains 'plugin' attribute in <files> children;
• check that module manifest contains either <element> tag or 'module' attribute in <files> children;
• check for required 'id' and 'type' attributes for package's <files> children;
• check that <files> node is not empty.