New template hooks to inject content from themes: Template.beforeTopBar, Template.beforeContent, Template.footer

[mattab]

We need new hooks to make Theming more powerful.

For example to:

• Modify the top bar functionality
• Add a footer
• Add content before the main content

Themes can use these template hooks to define new content. This was requested in: #255

[mattab]

In c3b6c38: Refs Pull request #255 Adding generic events to let themes inject content before the content, after the content, before the top bar
Fixes #4986