ViewPrivate: Make deselection by arrow keys more handy

Summary:
This patch move the cursor to the start or end of the selection
instead of simple one position into the key direction

BUG: 296500
FIXED-IN: 5.57

Reviewers: KTextEditor, dhaumann

Reviewed By: KTextEditor, dhaumann

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

Tags: Kate, Frameworks

Differential Revision: https://phabricator.kde.org/D19621