Fix typos, add formatting, consistent naming of drop-down lists
Summary: Fix typos, use :guilabel:, dropdown -> drop-down (as in the rest of KDE and in Wikipedia), drop down -> drop-down, colour -> color
Test Plan: none
Reviewers: Krita, Krita: Manual, woltherav
Reviewed By: Krita, Krita: Manual, woltherav
Tags: Krita: Manual
Differential Revision: https://phabricator.kde.org/D19949