Improved DX for Export UI

In building a bundle management UI for Fieldable Panels Panes (#1618308: User interface for managing bundles), I discovered a few places in the export-ui that could use some touch-ups. This is particularly in reference to the ctools_export_ui_process() function and ctools_export_ui() class:

• More array unions in defining $plugin['menu']['items']. This will allow developers to make subtle updates, such as just changing the 'type' of a menu item without having to define the entire menu item. This can certainly also be done by overriding the hook_menu() method, but then those modifications will be unknown to the plugin definition and will not be reference-able later.
• Array union in defining $plugin['redirect']. Currently, I cannot define new redirects or customize redirects because the export-ui process callback will clobber them.
• Define redirect paths for delete/revert. To stay consistent and avoiding hard-coded assumptions.
• Move the build of "allowed operation" links into its own method. This makes it easy to modify the "operation" links rendered, as well as cleans up the logic for removing inapplicable links.
• Move the deletion of exportable items into its own method. This makes it possible to do post-deletion processes by overriding the method. This also uses the redirect() method, instead of hard-coding a drupal_goto().