Add preference to check for availability of "Run utPLSQL test"

[PhilippSalvisberg]

Enabling and disabling the menu option "Run utPLSQL test" might be expensive. Especially when the context menu is called on a node of a schema with a lot of package specifications (the first time, but also the subsequent times). See #15.

The user may choose the enable/disable behaviour of the context menu in the utPLSQL preferences.

By default the "Run utPLSQL test" menu option should be available, even if no utPLSQL is installed in the database instance.