[cv6] Letter Index 1.2.5

• Listener: Added missing column existence guard in addonPostInstall for xf_rm_category.
• Forum Entity: Parameterized fetchLetterCounter() SQL query with prepared statement parameter (node_id = ?).
• Setup: Removed unused stub methods.
  [']Wording changed to letter to avoid confusion with a another AddOn,.

• Works now with XenFro 2.3.8+

XF 2.3 Version only

Forum and Category Settings are now divided into two options.
Enable indexing
Display Index

Added a Index Widget.

The indexing needs to be enabled that the widget is shown.

Updated the addon code to work with xf 2.3