For historical reasons, we had both StClickable and StButton, which
were nearly identical. StButton was more widely-used, so keep that and
port all StClickable users to that.
https://bugzilla.gnome.org/show_bug.cgi?id=640583
Current mockups display all search results as icons as used by
application results, so change the default result display to use
iconGrid/BaseIcon. Remove the custom application results display,
as it is no longer needed.
https://bugzilla.gnome.org/show_bug.cgi?id=634948
With the new layout, search results will be displayed in an independent
view like window previews, applications and possible future additions;
it does not make much sense keeping it with the switching logic, so move
the code to its own file.
Also remove the dash-prefix from the relevant style classes.
https://bugzilla.gnome.org/show_bug.cgi?id=634948
