XAR source projects should allow source files

This issue is to request the ability for project that uses the "xar" packaging to not only contain XML files but source files which the developer deems relevant to be included as such.

Examples include:

• javascript, velocity, css, or groovy text sources: they should have appropriate naming (.js, .vm, .css, .groovy) so that syntax colouring is enabled when sharing or editing
• resource files such as pngs or sounds so that they are displayed and edited with the appropriate tools (graphics editors, playable sounds, ...).

The XML of the source should "just" wire this inside to indicate which place the source fits in (attachments, object property, content, ...).

Example technologies to perform that include XInclude. Others are probably needed. This could be implemented in the XAR Mojo so that Wikis do not need to know about this feature which is for developers to work with their project.