Capital transformation for multiple keysyms

[wismill]

2a2a8d7 introduced the handling of Caps and Control keysyms transformations.

However, it works only for xkb_state_key_get_one_sym, but not for xkb_state_key_get_syms:

Also, only xkb_state_key_get_one_sym() is changed;
xkb_state_key_get_syms() is left as-is, and always reports the
untransformed keysyms.

I would like to revisit this decision:

This is for the following reasons:

• The API doesn't allow it, since we return a const pointer directly to
  the keymap keysyms table and we can't transform that.

We could use an internal buffer, such as for the Compose iterator (see xkb_compose_table_iterator::entry). As we discussed in #300, xkb_state is not meant to be thread-safe.

• The transformation doesn't make sense for multiple-keysyms.

I disagree: it feels quite a counter-intuitive result.

• It can be useful for an application to get the "raw" keysyms if it
  wants to (e.g. maybe it wants to do the transformation itself).

This is a valid point. But note that they are always available with xkb_keymap_key_get_syms_by_level.

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I see 2 directions:

1. Change the current API to support Caps transformation and use an internal buffer. This is a obviously a breaking change. Potential issue: we do not constraint the keysyms count, so what size should the buffer have?
2. Add a new API xkb_state_key_get_syms2 with a flag to set which transformation should be performed. We should probably let the user provide the keysyms buffer, much like xkb_state_key_get_utf8. Note that as stated above, we do not constraint the keysym count, so we should provide a feedback in case the buffer is too small.

Option 2 seems the safest.