Fix TableDelegate in DbExplorer properly

Before this commit, to change the state of the checkbox
you'd have to double-click and change the value manually
(to 1 or 0). This commit makes sure that you can click on the checkbox
and the change will propagate to the database.

There are still some issues:

• the row must be selected to change it/click on it
• user needs to click inside the checkbox to change its value
• double-clicking still gives the user textbox...