Merge branch 'sev/menu_sounds_reimplementation' into 'master'

Menu sounds reimplementation

This branch reimplements the menu sounds. It now plays different soundfiles for the following events:

OPEN: opening dialog
CLOSE: closing dialog
FOCUS: moving focus to new item
EXECUTE: pressing button, enter key, or double click
SELECT: using checkbox, radiobutton, or similar
SLIDE: using slider or colorpicker
CLEAR: clearing inputbox or keybind
WINNER: winning single player campaign

The soundfiles are moved into the dedicated directory *data/sound/menu*.

Unfortunately, I can't provide new soundfiles, since I have no experience with sound production,
so I've reused the legacy sounds or added placeholders.
However, I still think it is useful to merge this branch now,
because it improves the way that the sounds are played:

- It greatly reduces the "Geiger counter" effect of the focus sound when the cursor quickly moves across several items
- The focus sound plays for all interactive elements, e.g. also for the inputbox
- The execute (button) sound also plays for double clicks, e.g. in the gametypelist
- The audio settings tab gets a second checkbox to independently toggle focus sounds

See merge request !93