more convenient way to partially override list properties

the only way to edit the contents of list properties (that is, add or remove items) is to use the magic original property and js array functions. i'm not sure that's even possible on the command line (no example given in the documentation), but even if it is, it's extremely cumbersome. it would be way more convenient to have a syntax that covers the common cases, and at least for the cli (where it's most important) it would be reasonably easy to implement.

this issue came up (again) in QBS-1604.

possible syntax

• override (existing): foo.bar: baz
• remove: foo.bar:-: baz
• prepend: foo.bar:<+: baz
  • alternatively (shorter version): foo.bar:<: baz
• append: foo.bar:+: baz
  • alternatively (shorter version consistent with prepending): foo.bar:>: baz

the syntaxes using < and > obviously require quoting on the command line, which is an inconvenience, so the asymmetric version my be preferred (under the assumption that prepending is strictly required only in the minority of cases).

this syntax potentially conflicts with the csv syntax supported by list properties. however, it seems unproblematic to require quoting of punctuation characters, if that isn't even the case yet.