Decide where to install application & assets

[roblillack]

I have made a package in the AUR to make it easier for people to install. In case you are interested, I have made a small patch to the cmake definitions to use the lib/GNUstep/Applications/FSViewer.app directory instead of GNUstep/Apps/FSViewer.app since now GNUstep and WindowMaker applications go there (at least in Arch Linux).

Originally posted by @ogarcia in #7 (comment)

[roblillack]

Though FSViewer is not a real GNUstep application, I think it makes sense to install it alongside other such apps (like WindowMaker's WPrefs.app), which means:

• we need to figure out where to install it by default
• OS distributions should be able to change this prefix
• We should make ${PREFIX}/bin/FSViewer a symlink to ${PREFIX}/lib/GNUstep/Applications/FSViewer.app/FSViewer (or whatever) so the application can be started either way.

Revelant documentation seems to be the filesystem layout docs of gnustep-make.