Create a new mechanism to overwrite a skin file

The idea was first proposed there:XWIKI-10568 with a big discussion about this.

The original need was "Being able, in a Skin object, to overwrite a skin file placed in a subdirectory".

It seems that the cleanest solution is the one proposed by enygma: introduce a new mechanism: one XObject per file that we want to override.

The new XClass would be called "XWiki.XWikiSkinFileOverride".
It would contains 2 properties:

• a path (eg: htmlheaders.vm, less/buttons.less...)
• a content (a textarea).

We could also add a third property:

• merge mechanism, with 3 options: overwrite the original file, concat the original file with the new one by adding the new content before, concat the original file with the content by adding the new content after.

It would be good to also deprecate the current mechanism (modifying the XWiki.XWikiSkinClass or attach a file to the page).