keysyms: Revert removals and postpone effective deprecation

Recent removals & deprecations of keysyms have caused quite a lot of
trouble:
- Removed keysyms *were* used in the wild, especially the dead keys:
  e.g. in Neo-like and Bépo-like layouts.
  While checking for keysyms use in xkeyboard-config database is a
  mandatory first step, it is not enough.
- Removed keysyms broke Qt build.
- Deprecated names are now replaced by their new non-deprecated version.
  It is problematic in the case that a sandbox application (e.g. with
  flatpak) uses X11 or xkbcommon libraries older than the system’s one:
  they are not able to parse the new names.

For the sake of the stability, this commit restore all previous keysyms
and introduces the new names as alternative. While the comments mark
some names as “deprecated”, they will remain the canonical names (i.e.
they are *output* by `xkbcomp` and `xkbcommon`) for the next 10 years.

Details of the modified commits:
- "keysymdef.h: add better names for the dead schwa and SCHWA symbols"
  (commit e5cccecd814dd2005b3c5e9d22e426ff985ef387)
- "keysymdef.h: delete four symbolic names that serve no purpose"
  (commit cb44799b72f611eb4c9d7cc185bc3b09e070be08)
- "keysymdef.h: name the masculine ordinal indicator similar to feminine one"
  (commit 3ddfc9803676dd3b3ed909f8d9fbe1a699a5ce35)
- "keysymdef.h: add correctly spelled names for the left and right guillemet"
  (commit 8cfcc91a3f86cbea5ba676715412880ff6a61b21)

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