Fix #318388: Crash when trying to attach an image to key signature

[mattmcclinch]

Resolves: https://musescore.org/en/node/318388.

target->drop(data) should never be called unless target->acceptDrop(data) returns true.

[worldwideweary]

Although they approved this, it appears that this PR breaks image pasting generally speaking. I get crashes attempting to paste an image onto a notehead or a vertical frame, or even still a keysignature. Reverting this commit in @Jojo-Schmitz ( PR#9000 ) fixes the issue... so the idea was nice but it wasn't a thorough fix and made things worse, unfortunate to say.

[worldwideweary]

I take that back.. this is fine.. @Jojo-Schmitz screwed it up when bringing it into his PR #9000 which I'm trying to make use of since 4.x is lagging ... :-P I'll comment there

[Jojo-Schmitz]

I believe I see my screw-up (but that #7699 is not part of #9000, it is part of the 3.x branch which #9000 uses as its basis).

(If it isn't https://github.com/musescore/MuseScore/pull/7699/files#r822351432, let me know)