Cleanup in preparation of the next gtk-rs release

Closes #47, #46, #21, and #44

See merge request GNOME/gnome-tour!60

It is nowadays similar to the ImagePageWidget, so let us just use that

It is no longer used and if we ever re-add it, we can think of another way
to do that.

Fix last page button

See merge request GNOME/gnome-tour!62

(cherry picked from commit fc5a903f)

Fixes II

See merge request GNOME/gnome-tour!61

Instead of doing multiple overlays, do a single overlay containing all
buttons as type=overlay children. This makes the carousel span the
window from start to end.

This allow us to simplify the changes on the UI part

This would allow us to construct the pages on the ui files later

it doesn't make much sense now, but it will do once we port the rest of widgets as well

makes iteration on the UI slower

This is in preparation for a future gtk-rs release

it's part of the sdk nowadays

style: Stop hardcoding page colors

See merge request GNOME/gnome-tour!56

# Conflicts:
#   data/resources/style.css

Remove redundant and empty style classes

See merge request GNOME/gnome-tour!59