Clean up `plugin.xml`

[valich]

The current state of plugin.xml is dirty. It was created from some template and did not have any love since then. I think, we should:

• add a description with link to readme
• conceive versioning scheme and use it in the plugin
• decide whether we want to have changelog there or not.

[vladimir-koshelev]

I think that ide-plugin and arg-scanner should share a versioning scheme.

[valich]

@vedun-z I am not sure about that because it is likely that some part will become more-or-less stable (I predict arg_scanner will do) and the other will continue to evolve (at least we have some completely unresolved type-related problems there).

[vladimir-koshelev]

@valich We can use independent versioning, but plugin should depend on specific (probably major) version of arg-scanner.