Spanish translation (es_ES.pot) improvements

[Guillermo Welch]

There were some errors with the Spanish (es_ES.pot) tranlsation:

• Unsigned was translated as "Sin asignar" which means unassigned. The correct translation is "Sin signo".
• Empty table was translated as "Tabla vacía" which, instead of representing an action, represents the state of the table. Thus "Vaciar tabla" is the correct translation.
• On the same line as the previous one, Empty was translated as "Vacíos" which is a noun, not a verb. The correct translation is "Vaciar".
• Select all wasn't translated.

There where some other typos.