Closes prefereces table before opening the preferences form

This is a workaround for the following case:
* Open preferences of an alternative
* Create an alternative
* Open the preferences of the new alternative

As the preferences are created by means of a trigger the gvsig table doesn't know that a new record exists on the table. So, we close and open gvsig table always.
Fixes how the check for dem econ and dem centros is done