Respect users prefered text editor for shared text

Summary:
A user may have Kate installed but another editor set as default for text/plain. Respect that.

Also add .txt extension to the temp file name to make mime-type detection easier.

BUG: 399174

Test Plan:
Set default text editor to Kate -> Text opens in Kate
Set default text editor to Atom -> Text opens in Atom

Reviewers: KDE Connect, broulik, apol

Reviewed By: KDE Connect, broulik, apol

Subscribers: kdeconnect

Tags: KDE Connect

Differential Revision: https://phabricator.kde.org/D15813