Kate: Keyboard shortcuts F6/Shift+F6 for Next/Previous Match

Summary:
NOTE: depends on https://phabricator.kde.org/D17442

For the Search and Replace plugin use these default shortcuts as in Qt Creator:
- F6: Next Match
- Shift+F6: Previous Match

Remove unnecessary F keys:

- Remove default shortcuts F8 and F9 from the Replicode Plugin. F8 conflicts with a split view shortcut and F9 can stay free.
- Maybe also remove the F8 split view shortcut to make way for a non-F key variant

Reviewers: #kate, cullmann

Reviewed By: #kate, cullmann

Subscribers: cullmann, loh.tar, sars, dhaumann, kwrite-devel

Tags: #kate

Differential Revision: https://phabricator.kde.org/D17443