Feature request: Allow changing keybinding used to launch file manager

[dmccombs]

Issue description

Currently F1 is hardcoded as the key to open the PCman file manager utility from the UI on architectures that support it. On some hardware configurations it would be useful to remap this at run time to a different button than the default, or to disable it completely.

Please add support for specifying the key binding used to launch the file manager to the batocera.conf configuration file.