Adds a new highlighting effect to the actively selected row in
ComboBox ListView. ComboBoxEditor can now be controlled with
page up, page down, and the up and down arrow keys. ESC and loss
of focus now cause comboboxes to close. Now activates on mouseup
as well as return.
This is really just a workaround to keep SystemMonitor's process table
working right wrt selection retention during resorts (while also doing
full index invalidation on things like ProfileViewer inversion.)
It's starting to feel like the model abstraction is not super great
and we'll need a better approach if we want to actually build some more
dynamic functionality into our views.
AbstractView does not know which column it's displaying which makes it
impossible to implement the select_all functionality up there. Now
descendants override the pure virtual select_all method and implement
it themselves.
