Add support for 'css' prop

[undeadcat]

Adds support for 'css' prop. Fixes #69. Enables highlighting, completion for CSS values in strings passed to the 'css' prop - just like css, styled.div.

By default, the IDE doesn't know about 'css' prop, so CssPropAttributeDescriptorProvider is needed to include it in completion and avoid highlighting it as unknown.

To determine if css prop is available in project it currently checks that:

• styled-component package is installed and has version >= 4.0.0
• babel-plugin-styled-components is installed
• not checking .babelrc for plugin (at least for now) because babel options may be specified via command line, webpack config, .babelrc.js etc.

(It would be great if someone could confirm if these are the correct conditions for using 'css' prop? @mxstbr?)

(I'll update this with change notes and updated plugin.xml after #66 is merged. I'd like to release this separately since this updates the minimum IDE version to 2018.3 while #66 doesn't).

[thetruechar]

Definitely need this! Hope this coming soon!

[undeadcat]

@daedlock, Please have a look at this again. I'd like to merge this and release a new version. It seems css-prop is definitely a v4 feature since it has a v4 mark in the API description.

I've had to update some Gradle stuff (this affects plugin developers only) and updated the version and Changelog.