Changed "undo"  "next" and "clear" from icons to text buttons

No more red squares in black theme. (These icons for black theme were
missing.)

Button widths set manually to fit in on portrait mode. In order to fit
the entire "Next game" text that button is wider than others. (Just
"next" could be understood as "next player")
Undo disabled if nothing to undo
No longer deletea all sets if a set with empty name is deleted

QSettings::remove( const QString & key ) removes (as intended and
documented...) all keys if its given "" as the key. The user is allowed
to create a set with empty name (as should not have done). Since users
of earlier version may have created sets with empty name, disallowing
creation is not enough.

If a set with empty name is attempted to be deleted, all other sets are
read, all sets are deleted and other sets are written back.
Creating new sets with empty name prevented

Creating new sets with an empty string as the name is prevented. Just to
be on the safe side, as keys should never be empty in QSettings. The
problem with deleting such sets was solved earlier, this is to avoid
possible other problems...

Editing an old set with an empty string as the name is still allowed,
for user convenience, and the set name needs not to be changed for
saving. Saving with other name does not remove the original, so forcing
to change the name would not get rid of potential problems anyway.
Merge branch 'fremantle_emptysetnamesfix' into fremantle