Create an enumerated DocumentAction class for document actions (edit, view, download, etc)

Right now actions are strings retrieved with: "String XWikiContext.getAction()". This leads to:

• hardcoded strings in several places in the code. 23 occurrences for "download", 186 for "view", etc.
• fragile code. For example the code in ZipExplorerPlugin does: filename = getFileLocationFromZipURL(url, context.getAction().trim()); This is because there's always a risk of a white space somewhere...

I propose to create a DocumentAction class and to have DocumentAction XWikiContext.getAction(). The DocumentAction class would be implemented either using the commons-lang Enumerated model or using the model followed by Cargo. For example: http://fisheye.codehaus.org/browse/cargo/cargo/trunk/core/api/container/src/main/java/org/codehaus/cargo/container/ContainerType.java?r=987