[libcalamares] Make the zoneTab region list static

- there's only one zone.tab, it won't change during a run,
   load it only once (this allows for a single global list).

src/libcalamares/locale/TimeZone.cpp

@@ -130,10 +130,11 @@ TZRegion::~TZRegion()
     qDeleteAll( m_zones );
 }
 
-CStringPairList
+const CStringPairList&
 TZRegion::fromZoneTab()
 {
-    return TZRegion::fromFile( TZ_DATA_FILE );
+    static CStringPairList zoneTab = TZRegion::fromFile( TZ_DATA_FILE );
+    return zoneTab;
 }
 
 CStringPairList

src/libcalamares/locale/TimeZone.h

@@ -103,7 +103,7 @@ public:
      */
     static CStringPairList fromFile( const char* fileName );
     /// @brief Calls fromFile with the standard zone.tab name
-    static CStringPairList fromZoneTab();
+    static const CStringPairList& fromZoneTab();
 
     const CStringPairList& zones() const { return m_zones; }