Change icon entry in Thunderbird desktop file

By setting the "Icon" entry to "thunderbird" from "thunderbird.png" we
allow programs like rofi to detect the Thunderbird icon.

PR: 233591
Submitted by: Zach Dykstra <soporose@gmail.com>
Approved by: portmgr (blanket: run bugfixes)
MFH: 2019Q4
Differential Revision: https://reviews.freebsd.org/D22167