Fix #14576 : hiding courtesy time sig corrupts score

Fix #14576 : hiding courtesy time sig corrupts score

See http://musescore.org/en/node/14576 for discussion and examples.

1) Edited courtesy time sig (in any parameter: visibility, colour, text, ...) are saved to the score
2) When read back from file, such a time sig is not recognized as courtesy time sig
3) If the edited courtesy time sig no longer is at the end of a system, it is not removed.

The straight approach could have been to disable any editing of courtesy time sig. Rather than loosing this flexibility, I tried to improve the management of courtesy sigs:

1) not changed
2) when reading a time sig, now it is turned into a courtesy time sig if not at the beginning of a measure.
3) courtesy time sig removal now depends on its position in the system and not on the generated status.

Of course, if systems are re-arranged and an edited courtesy time sig becomes superfluous (no longer at the end of a system), it is removed and its edited parameters are lost: this is inevitable (otherwise a basic notation rule would be violated) and it is the responsibility of the user to make sensible modifications.

This patch also do the same changes, in the same code areas, for courtesy key signs, as they behave in a very similar way.