New feature: custom header and footer at workspace level

[ Feature description ]

At workspace level, an overall header or footer can be added.

It's defined by an URL, by a path to a local filename, or any
others supported by PHP protocols and wrappers.

It's displayed as is, not parsed or executed.

[ Goal ]

To allow workspaces to customized their environment

[ WorkspaceConfiguration class ]

• string WorkspaceConfiguration::$header Contains the URL or path to the header to print

• string WorkspaceConfiguration::$footer Contains the URL to path to the footer to print

[ Workspace configuration file ]

Two parameters are added to the JSON configuration files:

• header: the URL or path to the header to print (optional)
• footer: the URL to path to the footer to print (optional)

[ Views ]

• header.tpl: prints the custom header at first element of body
• footer.tpl: prints the custom footer at one of the last element of body, but before the JS libraries calls.

[ Notes ]

• This system is not intended to replace full customization. For that, create a new theme, and request as new feature a support of themes at workspace level.

• Login and error pages don't use header/footer views, so won't print them. For that, we should allow a workspace to replace some templates files by its owns.

[ Future plan ]

• Caching and caching invalidation of this feature (bug 24).
• Detect Smarty templates to offer access to variables (bug 25).