Part name in page header

The values $I, $i, and $:partName: are all defined as macros to return the part name for use in score headers (and footers). The corresponding value is defined as a score property only in extracted, linked parts, not in the main score. Thus these values return null when used in the header of a main score. To avoid confusion and provide better functionality the following two suggestions are offered:

1) Define partName as a score property in the main score, with a default value of "Full Score" or something similar (language dependent), so that value is returned by these macros when used in the main score.

2) Add $:partName: to the popup tooltip, as it is not listed there. ($I and $i are listed.) Also add it and an appropriate discussion to the online handbook.

If these are felt inappropriate then the tooltip needs to omit $I and $i in the main score, and the online handbook needs to explain the omission. $:partName: still needs to be added to the tooltip for the extracted, linked parts, and to the handbook.