services/glade: theme the header by looking at the 'toolbar' CSS class.

This provides consistency with many more GTK+ themes.
Some other design improvements.

base_ui: add spacing in the bottom-right buttons
welcome: reduce the arrow size to 48.
glade: switch status icons to icon names.

Since GTK+ 3.10, the stock icons are deprecated.
services/glade: remove set_header_deprecated().
glade frontend: default to 'info' icon name.