menu: add struct menu_parse_context to reduce static vars

menu: add struct menu_parse_context to reduce static vars

The lifetime of the "current_" variables (current_menu, current_item,
current_item_action) is very difficult to understand from reading the
code. It appears that e.g. current_menu could still point to a previous
menu when starting to parse a new one, with unpredictable results.

Let's use a context struct when parsing, and consistently initialize
it when beginning to build a new menu.

Lightly tested with:

- default menus (no menu.xml)
- example static menu from labwc.github.io/getting-started.html
- an added "client-list-combined-menu" sub-menu
- pipe menu generated by `labwc-menu-generator -p`

v2: style fix