[partition] Add setting for requiredstorage

- If the welcome module isn't used, can set a required amount
   of storage for the installation here.

src/modules/partition/gui/PartitionViewStep.cpp

@@ -69,6 +69,7 @@ PartitionViewStep::PartitionViewStep( QObject* parent )
     , m_widget( new QStackedWidget() )
     , m_choicePage( nullptr )
     , m_manualPartitionPage( nullptr )
+    , m_requiredStorageGiB( 0.0 )
 {
     m_widget->setContentsMargins( 0, 0, 0, 0 );
 
@@ -564,6 +565,9 @@ PartitionViewStep::setConfigurationMap( const QVariantMap& configurationMap )
 
     m_swapChoices = choices;
 
+    // Settings that overlap with the Welcome module
+    m_requiredStorageGiB = CalamaresUtils::getDouble( configurationMap, "requiredStorage", -1.0 );
+
     // These gs settings seem to be unused (in upstream Calamares) outside of
     // the partition module itself.
     gs->insert( "ensureSuspendToDisk", ensureSuspendToDisk );

src/modules/partition/gui/PartitionViewStep.h

@@ -86,6 +86,8 @@ private:
     QFutureWatcher<void>* m_future;
 
     QSet< PartitionActions::Choices::SwapChoice > m_swapChoices;
+
+    qreal m_requiredStorageGiB;  // May duplicate setting in the welcome module
 };
 
 CALAMARES_PLUGIN_FACTORY_DECLARATION( PartitionViewStepFactory )

src/modules/partition/partition.conf

@@ -120,3 +120,15 @@ defaultFileSystemType:  "ext4"
 #           % of the available drive space if a '%' is appended to the value
 #   - minSize: minimum partition size (optional parameter)
 #   - maxSize: maximum partition size (optional parameter)
+
+# Checking for available storage
+#
+# This overlaps with the setting of the same name in the welcome module's
+# requirements section. If nothing is set by the welcome module, this
+# value is used instead. It is still a problem if there is no required
+# size set at all, and the replace and resize options will not be offered
+# if no required size is set.
+#
+# The value is in Gibibytes (GiB).
+#
+# requiredStorage: 3.5