Add a Help item to the gear menu that opens the
node-format.md file in a new window. This could
be improved if we could parse markdown and apply
tags, similar to how we can load pango markup.
This commit is porting GtkPaned to be derived
from GtkWidget instead of GtkContainer, while adding
start-child and end-child properties. The existing
properties are renamed to follow the start/end naming
scheme, and we add proper getters and setters.
Update all users.
See #2719
Use window title, or custom title widget if it's set. Remove 'title'
property.
Update demos and tests to set the title on the window instead of
headerbar.
