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"
- using std::tr1::shared_ptr in namespace gui
- thus removing a lot of std::tr1 qualifications
- fix some includes. Should be relative to 'src'
- interface classes should declare a virtual dtor!
