We've refactored the code to support locating windows (and popups
windows + hot regions) correctly in both modes: with one ui::Display
and with multiple ui::Displays.
For this we've added a new ui::fit_bounds() function that works in
both modes.
This ui::Display is related to one native os::Window, so the dirty
region/invalid region is now part of a ui::Display instead of a
ui::Manager.
* Replaced ui::display_w/h() functions with ui::Display::size()
and ui::get_desktop_size()
* The ui::Manager contains the main ui::Display, and in the future an
ui::Window will have its own ui::Display
