libvirt-machine-properties: Add get_resources_properties()

Refactor the code that adds system resources (RAM and storage) into a
seperate method. We are declaring it as public API cause we'll be using
it from outside LibvirtMachineProperties in a following patch.

https://bugzilla.gnome.org/show_bug.cgi?id=741046
wizard: review() now public

We'll need to call it from WizardWindow in a following patch.

https://bugzilla.gnome.org/show_bug.cgi?id=741046
wizard-window: API to show resource customization UI

Add a view and API to show resources customization UI.

https://bugzilla.gnome.org/show_bug.cgi?id=741046
wizard: Make use of new customization view/API

Instead of launching properties view on user hitting 'Customize' button,
show the new resource customization UI. This means that now user can
only customize resources before hitting 'Create' button but that is fine
because resources are typically the only properties that users want to
customize before machine is launched for the first time.

Also this bebavior/UI is according to the new mockups.

https://bugzilla.gnome.org/show_bug.cgi?id=741046
wizard: Only LibvirtMachine is customizable

Now that customization during wizard only includes system resources, it
doesn't make sense to provide this option for any other type of boxes
than LibvirtMachine.

https://bugzilla.gnome.org/show_bug.cgi?id=741046
app-window: Remove redundant wizard->properties code

Remove now redundant code to launch properties from wizard.

https://bugzilla.gnome.org/show_bug.cgi?id=741046
wizard-toolbar: Keep a ref to WizardWindow

WizardToolbar now keeps an unowned reference to its parent WizardWindow.

https://bugzilla.gnome.org/show_bug.cgi?id=741046
wizard-toolbar: Content for customization view

The wizard topbar should provide a different set of controls and
title when customization view is visible in the wizard window.

https://bugzilla.gnome.org/show_bug.cgi?id=741046
libvirt-machine-props: Customization doesn't change UI state

The UI remains to be in wizard state when customizing now so the check
for UI state needs to be updated.

https://bugzilla.gnome.org/show_bug.cgi?id=741046
properties-toolbar: Drop main_back_button

The 'Back' button in main view was only needed for wizard->properties
case and now we don't have this UI transition anymore so this button is
now redundant.

https://bugzilla.gnome.org/show_bug.cgi?id=741046
properties-toolbar: Always show 'x' button

We didn't show 'x' (close) button when coming to properties from wizard
but now that UI transition doesn't happen. So there is no need anymore
to update the visibility of this button.

https://bugzilla.gnome.org/show_bug.cgi?id=741046
properties: Always start with 'General' page

We go straight to 'System' page if coming from wizard state but now we
never arrive from wizard to properties so there is no need for code to
switch to 'System' page anymore.

https://bugzilla.gnome.org/show_bug.cgi?id=741046
wizard: Always start with 'Introduction' page

We go straight to 'Review' page if coming from properties state but now
we never arrive from properties to wizard so there is no need for code to
switch to 'Review' page anymore.

https://bugzilla.gnome.org/show_bug.cgi?id=741046