it is a widely accepted rule to shape names with the usage site in mind.
Especially this means, that we use the singular form for all kinds
of collections and assortments.
Thus, the namespace should be called "widget" not "widgets",
because at usage site this becomes gui::widget::TimelineWidget
Likewise for "dialogs" and "pannels"
the rules are:
- our own headers go before any library headers
- all headers need to be spelled relative to include root
- ensure that gtk is always included via gui/gtk-base.hpp
