77b Update expressions

[michaelhkay]

This PR is the result of splitting PR #832 into two parts; this part extracts update expressions into a separate proposal, for ease of review.

[michaelhkay]

I have rebased the PR based on the current spec but there are some semantic inconsistencies that need to be ironed out. Known problems: There is a dependency on the selection-path() function which is not in the current spec, and the keywords used for modifiers in lookup expressions have changed.

[ChristianGruen]

In #832 (comment), I made a summary of some changes that I thought would need to be incorporated before map updates can be tackled. I’m copying them to this PR because I was looking them up more than once:

I think we would need a new version of this PR before we can look at #1283. Things I would expect to be added/revised:

• Singular forms of the keys/values/pairs modifiers.
• The clarification if we keep item or items, or use content.
• The note that arrays behave like map with consecutive integer keys.

…and various changes proposed for expressions.xml: https://github.com/qt4cg/qtspecs/pull/832/files#diff-b37a92a9eb3ab9ba48a00de9627a1124466b9c86ecb2b4989d04be3942c597a6