Instead of just the right-clicked item.

Rename m_session to m_last_value_session to better reflect its meaning
and distinguish it from m_current_value_session which will be introduced
in another commit.

Before, Dolphin would randomly crash when updating the cheat search when automatic refresh was enabled. (Having a large number of addresses listed, e.g. by starting with an "any value" search, may contribute). The crash was due to QTableWidget::item returning nullptr in RefreshGUICurrentValues, presumably due to the table being resized on the UI thread while the emulated CPU thread was updating the values. I've fixed this by pausing the CPU thread for the entirety of OnNextScanClicked; this eliminated crashes in my testing.

Move some #includes around to match the Contributing guidelines.

This is not every memory leak, just the ones that were obvious.