kcmodule: update combobox capitalization & fix singular form

The KDE HIG were recently updated to clarify guidance on combobox capitalization. Combobox text is now supposed to use sentence capitalization. Most comboboxes already follow this scheme, but the ones allowing the user to select the duration of inactivity do not.

This change updates the labels to conform to the new guidance.

HIG merge request at documentation/develop-kde-org!433 (closed)

I've also spotted an incorrectly set singular form for the duration settings in "Other Settings" and fixed that.

Screenshot before:

Screenshot after (showing both changes):