Disable Increase/Decrease buttons when limits are reached

And simplify the code:
- Use sal_uInt32 directly, without useless casting to SwTwips.
- Check for slot id directly, without an additional variable.
- Remove the redundant loop.

Change-Id: Ica76a166c88213cdd39e44482ce834d3555afb60
Reviewed-on: https://gerrit.libreoffice.org/7654
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>