Remember last used dirs for shortcut import/export

Summary:
File dialogs in KShortcutSchemesEditor use KoFileDialog instead of
plain QFileDialog. This allows to remember last used directories for
load/save/import/export.

Load, save and import location now defaults to homedir, instead of the
directory where Krita started.

BUG:368360

Reviewers: #krita, rempt

Reviewed By: #krita, rempt

Subscribers: rempt

Tags: #krita

Differential Revision: https://phabricator.kde.org/D19964