Enables pasting FiguredBass elements.

To be sure the element is pasted into the right track, the track is set again after reading the element.
This also affects Dynamic, Symbol, FretDiagram, Marker, Jump, Image, Text, StaffText, TempoText.
Merge pull request #236 from mgavioli/Figured_bass_enable_copy

FiguredBass: Enables pasting FiguredBass elements.