Fix deprecation warnings

[NathanBnm]

Here are the warnings I get when building the app from source. These are deprecation warnings and should be fixed:

../src/Widgets/SourceList.vala:1476.13-1476.24: warning: `Gtk.CellRendererPixbuf.follow_state' has been deprecated since 3.16
../src/Widgets/SourceList.vala:1607.74-1607.95: warning: `Granite.StyleClass.SOURCE_LIST' has been deprecated since 0.4.2. Use Granite.STYLE_CLASS_SOURCE_LIST
../src/Widgets/SourceList.vala:2327.25-2327.34: warning: `Gtk.Menu.popup' has been deprecated since 3.22
../src/Widgets/SourceList.vala:2329.25-2329.34: warning: `Gtk.Menu.popup' has been deprecated since 3.22
../src/Widgets/SourceList.vala:2631.9-2631.28: warning: `Gtk.Widget.push_composite_child' has been deprecated since 3.10
../src/Widgets/SourceList.vala:2633.9-2633.31: warning: `Gtk.Widget.set_composite_name' has been deprecated since 3.10
../src/Widgets/SourceList.vala:2634.9-2634.27: warning: `Gtk.Widget.pop_composite_child' has been deprecated since 3.10