tdf#117066 Saving ODT document with ~1500 bookmarks is slow, part 5

Individually, these don't make much difference, but they add up
to a halving the time to save on my machine.

OStorageImpl is spending time iterating over its m_aChildrenVector to
find stuff by name, so just use a std::unordered_map.
Also return iterator from FindElement, so we can avoid searching the map
twice.
This was probably the biggest win.

Change-Id: I30776bad5377d14144fc7a47e86527e2cdb62a83
Reviewed-on: https://gerrit.libreoffice.org/70313
Tested-by: Jenkins
Reviewed-by: Noel Grandin <noel.grandin@collabora.co.uk>

package/source/xstor/xstorage.hxx

@@ -139,7 +139,7 @@ struct OStorage_Impl
         return m_nModifiedListenerCount > 0 && m_pAntiImpl != nullptr;
     }
 
-    SotElementVector_Impl                         m_aChildrenVector;
+    std::unordered_map<OUString, SotElement_Impl*> m_aChildrenMap;
     SotElementVector_Impl                         m_aDeletedVector;
 
     css::uno::Reference< css::container::XNameContainer > m_xPackageFolder;
@@ -205,7 +205,7 @@ struct OStorage_Impl
     void ReadContents();
     void ReadRelInfoIfNecessary();
 
-    SotElementVector_Impl& GetChildrenVector();
+    bool HasChildren();
     void GetStorageProperties();
 
     css::uno::Sequence< css::uno::Sequence< css::beans::StringPair > > GetAllRelationshipsIfAny();
@@ -229,6 +229,7 @@ struct OStorage_Impl
                             bool bDirect );
 
     SotElement_Impl* FindElement( const OUString& rName );
+    std::unordered_map<OUString, SotElement_Impl*>::iterator FindElementIt( const OUString& rName );
 
     SotElement_Impl* InsertStream( const OUString& aName, bool bEncr );
     void InsertRawStream( const OUString& aName, const css::uno::Reference< css::io::XInputStream >& xInStream );
@@ -242,7 +243,7 @@ struct OStorage_Impl
 
     css::uno::Sequence< OUString > GetElementNames();
 
-    void RemoveElement( SotElement_Impl* pElement );
+    std::unordered_map<OUString, SotElement_Impl*>::iterator RemoveElement( std::unordered_map<OUString, SotElement_Impl*>::iterator pElement );
     static void ClearElement( SotElement_Impl* pElement );
 
     /// @throws css::embed::InvalidStorageException