build: Reinstate symlinks from legacy names to closest CSS names

• build: Reinstate symlinks from X11 cursor names to closest CSS names

  Before commit 74e9b794 "cursors: only use gtk/css names", each cursor was available under several names: the traditional/legacy X11 cursor font name (which was treated as canonical), the CSS/freedesktop.org name if any, and various other traditional/legacy names.

  Eliminating the non-CSS cursors such as dotbox doesn't mean we can't still provide the traditional X11 cursor font names as aliases for the CSS/freedesktop.org names (which are now treated as canonical). This considerably improves interoperability with non-GTK toolkits and applications, some of which have historically looked up cursors by their X11 names: for example, Firefox before 123 and SDL before 2.30.1 did this.

  The non-cursor-font-based aliases that were present in Adwaita 45 (additional traditional/legacy names possibly originating from Qt such as size_fdiag, and hash-based libXcursor names such as c7088f0f3e6c8088236ef8e1e3e70000) have been excluded here, as requested by Jakub Steiner in !65 (merged).

  Fixes: 74e9b794 "cursors: only use gtk/css names"
  Resolves: #278 (closed)

• build: Reinstate more traditional aliases for cursors

  These are not part of the X11 cursor font, but were set up by Adwaita 45 and older. Many of them seem to be Qt inventions.
  — Dropped as requested

• build: Reinstate hashed aliases for cursors

  X11 desktops traditionally had an ad-hoc mechanism for theming custom cursors set by applications: libXcursor hashes the bytes of the custom cursor, converts the hash to hexadecimal and looks up a cursor of the same name in the icon theme. Restore the hash-based aliases that existed in the Adwaita theme prior to commit 74e9b794.
  — Dropped as requested