Enable Text Filter plugin by default

Summary:
The Text Filter plugin is by far one of Kate's most useful and powerful plugins, and when enabled, helps to distinguish Kate from more common text editors and put it in league with the really powerful ones like Sublime Text on Linux and Textmate on macOS.

However, the plugin is disabled by default! Considering that Kate is a power user text editor, I think having it enabled by default is more appropriate. This will improve the feature's discoverability and make Kate even more powerful for its target audience.

Test Plan: Open Kate with a new user account and see that the menu item is enabled under the {nav Tools} menu

Reviewers: #kate, dhaumann, cullmann

Reviewed By: #kate, dhaumann

Subscribers: sars, kwrite-devel

Tags: #kate

Differential Revision: https://phabricator.kde.org/D14414