[Mod/Fix] No longer infer a key command's input context from either t…

…he key config dialog or from the contents of MKB files. Reassign command input context whenever a command is added to the key map instead. This avoids commands ending up in the wrong key context in case they are read from a malformed MKB file, and avoids bugs like the one fixed in r22257.

git-svn-id: https://source.openmpt.org/svn/openmpt/trunk/OpenMPT@22303 56274372-70c3-4bfc-bfc3-4c3a0b034d27