Allow keyswitches to be grouped for keyswitch dropdown

[alcomposer]

Keyswitch groups allows sfizz (or any SFZ player that wants to support this feature) to display group sub-menus in the keyswitch dropdown planned for Panel B:

This will allow library authors to categorise their keyswitches how they want, and facilitate ease of discovery for both author, and end user.

In order to allow this it is suggested that the current sw_key_labelN use a | character to divide the string name into two parts:

sw_key_labelN=<keyswitch group name> | <keyswitch name>

SFZ Example:

<control>
sw_key_label42=instrument | sine
sw_key_label43=instrument | saw
<region>
sample=*sine
sw_last=42
<region>
sample=*saw
sw_last=43

Looking at the above example, this solution could also remove the need for adding the keyswitch name per region.

The keyswitch group name metadata would also be useful to display above the sfizz virtual keyboard. For both users, and lib makers:

Can also be expanded to work with multi-keyswitch in future: #21