[partition] Update next-button

You'll need a VM with 2 disks to demonstrate:

- Configure Calamares to pick "none" as initial action on
  the partition page (this is a safe choice),
- Enter partition page,
- No action is selected, and the next> button is greyed out.
- Click erase; notice next> is now available.
- Change devices, notice no action is selected, but next>
  is still available. Clicking on it, though, does nothing.

When changing to "no action", update the next-button's
availability.

src/modules/partition/gui/ChoicePage.cpp

@@ -517,6 +517,7 @@ ChoicePage::applyActionChoice( InstallChoice choice )
         break;
     case InstallChoice::NoChoice:
     case InstallChoice::Manual:
+        updateNextEnabled();
         break;
     }
     updateActionChoicePreview( choice );