Allow multiple PWADs to be selected without an IWAD

Previously, only one PWAD could be selected in the WAD launcher without
an IWAD, and DOOM RETRO would automagically look for it. Now, inspired
by the fact Back to Saturn X: Episode 2 is split into 2 PWADs,
BTSX_E2A.WAD and BTSX_E2B.WAD, more than one may now be selected.