[Linux] force to disable 'Popup fade-in' option

[gomita]

Currently, 'Popup fade-in' option is disabled forcibly on Linux because of some graphical problem. It will be enabled when xul:panel[type="arrow"] will support fade-in effect on Linux. I should watch: http://mxr.mozilla.org/mozilla-central/source/toolkit/themes/linux/global/popup.css