fix #34896: header/footer macros $F and $f

show bogus information for linked parts, a non-existing path resp.
filename.
Let's use rootScore()'s name instead.
fix #34421: header/footer macros $M and $m

don't work in linked parts (show empty).
Let's use rootScore()'s tdate and time instead.
Also once a score has been modified, hence marked dirty, these marcos
don't reflect that.
Let's use current date and time in this case.

Remaining issue: having changed score/part and saved it, these macros
still don't reflect the change, the file needs to get closed and
reopened. Here some method to refresh this fileInfo() data would be
needed.
add a $N macro to footer/header to show page number

on page one only if there is more than one page in the score/part
refresh QFileInfo after having saved a score

and update() the display too
fix typo
Merge pull request #1344 from Jojo-Schmitz/last-modified

fixes and an extension to the header/footer macros