R1832: Load memory table

Old implementation of MemTable loadTable() is not correct:
- If memtable is backed by m_data, the table is not refreshed.
- If memtable is not backed by m_data, table refreshed by real data are not updated.

I reimplemented MemTable's loadTable() to make it efficiently work. Only cells that changed are refresh/updated.

toIntVector() return a QVector. Since QVector uses implicit data sharing, copy a QVector is not an heavy operation.   (see https://doc.qt.io/qt-5/implicit-sharing.html)

Thank you.
Patch applied at Rev 1834.