We were not taking the width of the process headers into account when
computing the scrollable content size of the timeline.
Fix this by passing the header width to AbstractScrollableWidget's
set_size_occupied_by_fixed_elements().
The architecture here is a little bit convoluted. I ended up making a
new container widget (TimelineContainer) that works similarly to
GUI::ScrollableContainerWidget but has two subwidgets (a fixed header
that only scrolls vertically, and the timeline view that scrolls on
both axes.)
It would be nice to generalize this mechanism eventually and move it
back into LibGUI, but for now let's go with a special widget for
Profiler so we can continue iterating on the GUI. :^)
