Add support for disk control interface v1 (disk display labels)

[jdgleaver]

This PR adds support for libretro disk control interface v1 (c.f. libretro/RetroArch#10019) - in particular, it enables the display of disk 'labels' when using RetroArch's Current Disk Index drop-down list to select disk images.

By default, disk labels are set to the image filename without extension - but when using M3U playlists custom labels may be set via the following syntax: DISK_FILE|DISK_LABEL. For example, this M3U file:

Valhalla & the Fortress of Eve_Disk1.adf|Game Disk
Valhalla & the Fortress of Eve_Disk2.adf|Data Disk
Valhalla & the Fortress of Eve_Disk3.adf|Level 1 Disk
Valhalla & the Fortress of Eve_Disk4.adf|Level 2 Disk
Valhalla & the Fortress of Eve_Disk5.adf|Level 3 Disk
Valhalla & the Fortress of Eve_Disk6.adf|Level 4 Disk

...will produce the following menu:

If DISK_LABEL is intentionally left blank (i.e. DISK_FILE|) then only the disk index will be displayed.

Save disks generated by the #SAVEDISK: keyword are automatically assigned the label: SAVE DISK [SAVE_DISK_INDEX].

Here are some more examples:

HeroQuest - Return of the Witch Lord_Disk1.adf|HeroQuest
HeroQuest - Return of the Witch Lord_Disk2.adf|Expansion - Return of the Witch Lord
#SAVEDISK:
#SAVEDISK:

Sensible Golf (1995)(Virgin)(Disk 1 of 2).adf
Sensible Golf (1995)(Virgin)(Disk 2 of 2).adf
#SAVEDISK:

Secret of Monkey Island_Disk 1.adf|
Secret of Monkey Island_Disk 2.adf|
Secret of Monkey Island_Disk 3.adf|
Secret of Monkey Island_Disk 4.adf|
#SAVEDISK:

[inactive123]

Hi there @rsn8887, I think this is a good PR to merge but I'll leave the final reviewing up to you.

[rsn8887]

Looks good thanks!