Changed, action argument inputs: "+++" and "---" prefixes now do the same thing as they do in other numeric controls (e.g. add/subtract given value multiplied by selected map element number to/from initial value).
Changed, action argument inputs: changed value colors, added tooltips.
Added two new type handlers that can be used in UDMF ('Thing Type' and 'Thing Class')
Polished the browse icons and browse buttons.
Added more events for plugins to intercept.
