Allow to copy or move selection to the other split view

Summary:
FEATURE: 276167

Default keyboard shortcuts set to `SHIFT+F5` for `copy`, `SHIFT+F6` for `move`

Reviewers: #dolphin, elvisangelaccio, ngraham, meven, dfaure

Reviewed By: #dolphin, elvisangelaccio, ngraham, meven, dfaure

Subscribers: yurchor, kde-doc-english, dfaure, meven, kfm-devel

Tags: #dolphin, #documentation

Differential Revision: https://phabricator.kde.org/D29006