Generate Unicode↔Keysym tables

[aperezdc]

It would be desirable to generate unicode_to_keysym_table and keysym_to_unicode_table from a single source e.g. using a script, to avoid them having different or missing entries—for example, see #53 for an example of such a situation.

[aperezdc]

We may be able to use keycodemapdb for this ⌨️