ResourceLoaderLessVarFileModule: Add parameter 'lessMessages'

ResourceLoaderLessVarFileModule: Add parameter 'lessMessages'

This change allows to add internationalized messages into CSS. The new
parameter 'lessMessages' contains the array of message keys that get
set as Less variables.

In Less the variables must enclosed in double quotes (") or single
quotes (') to prevent CSS injection.

Example usage:
Resources:
	"class": "ResourceLoaderLessVarFileModule",
	"lessMessages": [ "editsection" ],
Message 'editsection':
	edit
Less file:
	content: '[@{msg-editsection}]';
Generated CSS file:
	content: '[edit]';

Also add a type case (object) to ensure that an empty object is
serialized as '{}' and not as '[]'.

Also include Less variables from parent.

Change-Id: I280b52c6745fe8e5755dc5d58c5621a64757d99d