This patch adds GUI::Action::create_checkable() helpers that work just
like the existing create() helpers, but the actions become checkable(!)
Clients are no longer required to manage the checked state of their
actions manually, but instead they will be checked/unchecked as needed
by GUI::Action itself before the activation hook is fired.
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.
If the cursor left a table view column header while also pressing it,
we would keep the header highlighted. This was not consistent with how
regular buttons behaved.
