API: SQL class definitions renaming
WIP, related to #7960
- [x] renaming of 
  - PandasSQLAlchemy -> SQLDatabase
  - PandasSQLTable -> SQLTable
  - PandasSQLLegacy -> SQLiteDatabase
  - PandasSQLLegacyTable -> SQLiteTable
  - SQLDatabase.read_sql -> read_query
- [ ] adding docstrings to public functions of SQLDatabase
  - [x] read_query, read_table, to_sql
  - [ ] has_table, drop_table, ..
- [ ] add docs on use of SQLDatabase
- [ ] add api docs for SQLDatabase
- [x] rename some mentions of 'legacy' to 'fallback'

@jorisvandenbossche we sched to remove the old sql support in what 0.16?
 Should I add deprecations for the old names? (they were never really public, so for now I just removed the old names)

I am going to leave the docs part of this PR for later (can do after rc release) 
For the removal of old sql support, I don't think we explicitely scheduled that, but I would say 0.16 or 0.17.
 ok on old sql support (add to the deprecations list if not already on it)

If these were not public then I don't think a big deal (you can add a doc note if you want)
