Install icon and .desktop file during make install

Makes packagers' lives easier, looks better and ensures Julia
will show up in software centers like GNOME Software.

(cherry picked from commit b382a743e6e7b00e8a375b9b722b27cd9f5e7168)
ref: #8425
Add appdata file

(cherry picked from commit b296f6aaaf06c6517c74c48536f58f48bf53abba)
ref: #8610