Fix org.telegram.desktop identification

Summary:
The code we have that adds .desktop at the end of the resource name did not take
into account that it might be the case that the application ends with .desktop
like it happens on telegram's desktop client

This patch looks for the file instead of just checking the name to account for
it. Otherwise the look up happens in KConfig.

Fixes flathub/org.telegram.desktop#27 on github

Test Plan: Tested locally, works.

Reviewers: Plasma, KWin, ngraham

Reviewed By: ngraham

Subscribers: kwin

Tags: KWin

Differential Revision: https://phabricator.kde.org/D29728