Allow specifying DOSBox location manually

[JaapJoris]

Currently, IA Launcher tries to launch DOSBox by calling the dosbox command and by searching some default installation directories. However, the user should be able to specify a custom DOSBox installation path.

[DaVyper]

additionally it shouldn't have to be dosbox.exe/etc either as maybe the user likes dosbox-x or dosbox-staging and so on and shouldn't have to rename just to make it work