desktop-entry: better handle desktop files with dots in their name

desktop-entry: better handle desktop files with dots in their name

This fixes - among others - cases like `R.E.P.O..desktop`.

Due to the duplicated dot at the end we were matching against
a 0 sized string which was always true and thus would always
match the desktop file, regardless of what the app-id was.

A related issue is that an app-id of `osomething` would match
a desktop file name called `R.E.P.O.desktop` due to the string
being size of 1.

Fix this by requiring the partial desktop filename string to
be at least 3 characters wide.

Fixes #3257