Factor out iteration over journal entries
Add applications category filter
Add hardware category filter
Add security category filter
Mark updates and usage categories as unimplemented
Add important category filter

Additionally, mark alerts and starred categories as unimplemented.
Use GtkHeaderBar
Set image-button style class on toolbar buttons
Use two header bars for the titlebar
Stop using libgd

As the toolbars are now using GtkHeaderBar, there is no need to use
GdMainToolbar.
Look up action strings from generated GEnumValues