KateSpellCheckDialog: Remove action "Spellcheck Selection"

Summary:
...and add logic to "Spelling.." and "Spelling (from cursor)" actions
when there is a selection.

This way the behavior fit to other actions where the selection as
precedence over some default target and we have one entry less in menus.

Reviewers: KTextEditor, cullmann

Reviewed By: KTextEditor, cullmann

Subscribers: cullmann, dhaumann, kwrite-devel, kde-frameworks-devel, KTextEditor

Tags: Kate, Frameworks

Differential Revision: https://phabricator.kde.org/D18663