Disambiguate various ‘New’/‘New...’ strings for translators

For some languages, the translation of the word ‘New’ depends
on which type of ‘new’ object that is created (e.g. on the gender
of the word ‘xxx’ in the string ‘New xxx’). This commit adds
message context (msgctxt) to disambiguate the various ‘New’/‘New...’
strings.

It also fixes a few inconsistencies in the use (or not) of ‘...’
and the placement of the ‘&’ accelerator marker.