For toggle icons, the icon should stay the same

[simonharrer]

JabRef version JabRef 3.7dev--snapshot--2016-09-14--master--ff7c9b2 windows 10 10.0 amd64 Java 1.8.0_102

Steps to reproduce:

1. Open JabRef, see Toggle Global Search with strike-through globe
2. Toggle button
3. Globe is activated, but the icon changed to a normal globe

I would expect that if it is active, the toggle status already shows this, and the icon stays the same. Hence, I would propose to just use the globe.