Merge modes big refactor

[wismill]

Follow-up of #568, with tests and fixes for all components.

Merge modes parameters are aggressively removed, so that:

• Sections are always compiled in isolation
• Merge mode for inclusion does not affect the included section until merging with either the previous section in the include statement or its parent.
• … ?

See this comment from @mahkoh and the follow-up discussion.

TODO:

• Rebase after xkbcomp: Fix key merge mode (keysyms, actions) #568 is merged.
• It seems explicit mapping of virtual modifiers is handled poorly when dumping the keymap. See Fix explicit vmod dump #595.
• Disallow merge mode propagation upwards? At least have it consistent across all components.

[wismill]

This can potentially break some advanced setups that rely on merge modes current propagation, so maybe it is safer to wait for next release while improving the tests.