(Breaking changes for) docxml v6

[Wybe]

Collecting thoughts for the next breaking change in docxml's API:

• Fix the naming inconsistency between borders.left/borders.right (tables) and borders.start/borders.end (table cells)
• Drop indentation.hanging, since it is just the inverse of .firstLine. A negative value for .firstLine is the same as a positive value for .hanging, and they are therefore mutually exclusive
• Support relative values for @line paragraph spacing