Improve the discoverability of the Filter Text plugin and its menu item

Summary:
The "Filter Text" plugin is one of Kate's most useful plugins for serious professional use, but it's rather hidden behind a nondescript name, caption, and menu item.

This patch improves the caption to mention "terminal commands" to give a hint about its true power, and also uses a compact variant of that same terminology for the menu item. These changes should improve discoverability.

BUG: 396864
FIXED-IN: 18.12.0

Test Plan:
More descriptive caption in the plugins list:

More descriptive menu item text:

Reviewers: Kate, dhaumann, cullmann

Reviewed By: Kate, dhaumann, cullmann

Subscribers: kde-doc-english, lueck, kwrite-devel

Tags: Kate, Documentation

Differential Revision: https://phabricator.kde.org/D14390