[partition] Sample of requirement checking

- There should be at least device to select, overwise we can't continue.
   The check currently **always** fails, so it blocks installation.

src/modules/partition/gui/PartitionViewStep.cpp

@@ -579,5 +579,21 @@ PartitionViewStep::jobs() const
     return m_core->jobs();
 }
 
+Calamares::RequirementsList PartitionViewStep::checkRequirements()
+{
+    Calamares::RequirementsList l;
+    l.append(
+        {
+            QLatin1Literal("partitions"),
+            []{ return QString(); },
+            [this]{ return tr("There are no partitons to install on."); },
+            false,  // satisfied
+            true    // required
+        });
+
+    return l;
+
+}
+
 
 CALAMARES_PLUGIN_FACTORY_DEFINITION( PartitionViewStepFactory, registerPlugin<PartitionViewStep>(); )

src/modules/partition/gui/PartitionViewStep.h

@@ -66,6 +66,8 @@ public:
 
     QList< Calamares::job_ptr > jobs() const override;
 
+    Calamares::RequirementsList checkRequirements() override;
+
 private:
     void initPartitionCoreModule();
     void continueLoading();