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 (and xserver, but the corresponding commit was reverted).
• 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. See e.g. “Can't input «angle quotes»”

For the sake of the stability, this PR 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" (!71 (merged))
• "keysymdef.h: delete four symbolic names that serve no purpose" (!70 (merged))
• "keysymdef.h: name the masculine ordinal indicator similar to feminine one" (!68 (merged))
• "keysymdef.h: add correctly spelled names for the left and right guillemet" (!68 (merged))

CC @whot