wizard: Make page setter public

In a following patch, we'll introduce a new widget that will need to
take charge of switching wizard pages.
Put wizard in a modal dialog

With some UI transitions removed during the port away from clutter, its
better to have it in a dialog. This also helps reduce the size of wizard
to buttons on both sides are not so far away from each other and hence
easy to reach.
Drop now redundant WizardToolbar
wizard: Remove now redundant cancel_button field
Put properties in a modal dialog

With some UI transitions removed during the port away from clutter, its
better to have it in a dialog.
Drop now redundant PropertiesToolbar
properties-sidebar: Adjust spacing & margins

After getting moved into the new properties dialog, we need to adjust
some spacing and margins to look good.
libvirt-machine-props: troubleshoot transient over props dialog

"Troubleshooting log" dialog should now be transient over the new
properties dialog rather than app window.
Drop now redundant Sidebar class
tmp