kcms/rules: use sentence case for combobox, apply small wording fixes, and replace panel icon

The HIG recommend sentence case for combobox entries. The comboboxes in the rules kcm currently mostly use title case for their entries. The property names mostly use sentence case, although there are few exceptions using title case. The HIG are not explicit here (although sentence case seems to be in the spirit of the guidance); in any case it should be consistent.

This change converts the labels and property names to sentence case. It also includes some minor phrasing touch-ups, in particular regarding on- screen displays which are now spelled correctly and consistently.

In addition, the icon for matching the panel window type, now ill-fitting as list-remove was changed to a red X instead of a minus, is temporarily replaced with spinbox-decrease until a proper panel icon is ready (bug 492341).