osx_common: cache OSX version number

This commit makes `is_osx_version_at_least` cache the result of reading
`/System/Library/CoreServices/SystemVersion.plist`. Since that is a file
read operation it was bad to use this function frequently (i.e.: when
processing user events).

Remove `is_lion_or_above` (introduced in c9396c0a) as that was a more
specialized wrapper which had the only advantage of adding it's own cache.

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