texture-cache: require icon scale to load gicon

To support HiDpi.

https://bugzilla.gnome.org/show_bug.cgi?id=705410
shell-app: finish support for loading with scale

https://bugzilla.gnome.org/show_bug.cgi?id=705410
altTab: don't override icon size request

Instead of overriding the actor's request with the icon size, just set
the new icon size on the actors, and let the default handler take the
preferred size of children.

https://bugzilla.gnome.org/show_bug.cgi?id=705410
altTab: account for scale factor when computing icon size

https://bugzilla.gnome.org/show_bug.cgi?id=705410
iconGrid: don't force icon size to the BaseIcon

Since we might get a scaled up version for HiDpi.

https://bugzilla.gnome.org/show_bug.cgi?id=705410
st-icon: remove custom size request/allocate

Use a layout manager instead. This has the effect of not enforcing a
priv->icon_size size request, since that value is unscaled.

https://bugzilla.gnome.org/show_bug.cgi?id=705410
dash: account for scale factor to determine icon size

https://bugzilla.gnome.org/show_bug.cgi?id=705410
appDisplay: don't force size on folder icon actor

Use a table layout instead.

https://bugzilla.gnome.org/show_bug.cgi?id=705410