Add page templates

[zerocrates]

Imported from Trac ticket #32 reported by jboggs on 2009-07-25:

Give SimplePages the ability to use page templates for individual pages. Page templates could show up in a select menu on the page building form, and users could select a template to use.

[zerocrates]

Comment by jboggs on 2011-01-20:

This could add a dropdown of file names from current theme. If the user switches the theme, it would need to loop through each page, and switch to the default template if the template assigned to the page does not exist in the new theme.