APP_DRIVE and APP_DRIVE_LETTER config variables

[polyvertex]

Add the APP_DRIVE and APP_DRIVE_LETTER predefined configuration variables in the [var] section which respectively must expand to U:\ and U if Keypirinha is installed in U:\apps\Keypirinha for example.

Ref: initial discussion

Edit: stick to adding APP_DRIVE only. APP_DRIVE_LETTER would be empty in case of a network path and it can be harmful to have it expanded to an empty string.

[polyvertex]

APP_DRIVE predefined configuration variable is now available in v2.16.
Usage: ${var:APP_DRIVE}