Set preferred height instead of fixed height to allow the splitter to
move.
The splitter respects set_fixed_height() after a808cfa7 "LibGUI+
Applications: Govern Splitter resizing by opportunistic growth", and
that caused the splitter in Profiler to stop working.
This reverts commit cfef3040fb.
It looks like although this does improve things, it also degrades the
experience and messes with the usability, especially for large amounts
of processes.
Need to come back to this with a more holistic fix.
Today the profile viewer timeline view has a static size, which is
computed as half the height of the window given it has two root widgets.
Instead the timeline view should shrink to only consume the size that
each process timeline consumes.
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. :^)
