provide a way for cores with need_fullpath==false to determine content filename

Many cores create save and configuration files which are named based on the filename of the content they are executing.

When a core sets need_fullpath==false, there is no means specified in libretro.h for the core to determine the filename so there is no way to name the save files. @fr500 provides an example of this issue from beetle-gba here: https://github.com/libretro/beetle-gba-libretro/pull/33#issuecomment-406910261

I'm creating this issue because it seems separate from the PR I submitted to clarify the existing behavior.

Two possible ways I can think of to provide the name of the content to the core are:

• update the API so that if need_fullpath == false then retro_game_info::path will return the filename of the content (not the full path)
• create a new callback which returns the name of the content

Start with libretro.h and the definitions of retro_game_info, need_fullpath, and existing callbacks. Compare the two API options described in the issue with the linked PR clarifying existing behavior; done means cores can determine the content filename when need_fullpath is false through an agreed API behavior.