install FileManager.h too
In IRC today, two people asked if it was expected or not that FileManager.h was not installed:
(20:23:30) l3u: Dear marble devs, my Gentoo installation of Marble (20.04.3) lacks the FileManager header. Do you have any idea why or how to get it?
(20:50:34) jzarl: hi! I noticed that the FileManager header file is not installed by make install - is this a conscious decision or an accident?
In case it is an accident, this MR should install it.