Merge src/filemanager/option.[ch] to src/filemanager/boxes.[ch].

Signed-off-by: Andrew Borodin <aborodin@vmail.ru>
FIXFOR: (subshell_prompt): changed to GString.

Signed-off-by: Andrew Borodin <aborodin@vmail.ru>
Clarify usage of use_internal_edit variable

...and disable "Use internal edit" checkbox in case of build
without internal editor.

Signed-off-by: Andrew Borodin <aborodin@vmail.ru>