TSQLite-classes to access sqlite3-databases. See ROOT-5256.

The attached patches add the following classes:
TSQLiteResult
TSQLiteRow
TSQLiteServer
TSQLiteStatement
and corresponding integration into both the ROOT-buildsystem and cmake.
Naturally, also the macro for the plugin-manager is included.
Patches supplied for ROOT 5.34.08 and current master (as SQL-part is
rarely changing, this statement should stay valid at least for some months).

Some usage notes:

* The handed path is evaluated as URI by sqlite3, which means it can be
  a relative or absolute path and can even contain additional settings
  as "URI" parameters, which are described in the SQLite-documentation.
* Functions "{Create,Select,Drop}
  DataBase(s)", "Reload", "Shutdown" are not implemented as SQLite
  operates on one DB per file only. The implemented stub methods return
  appropriate error results.
* "TSQLiteServer::GetColumns()" is not implemented (i.e. returns error
  and value 0): This is sadly not possible via a query in SQLite by
  their API design. This also means the TableInfo-objects cannot be queried.
* The result-row-count of a TSQLiteResult is always -1, as SQLite does
  not know the number of result rows before all have been retrieved
  (a valid optimization for a file-only DBMS).

For the person reviewing this patch, please note that the indices of
bind_parameters in SQLite start from 1, while those for the result-columns
start from 0.

As this is a rather large featureset to cover and the software of our
experiment currently only uses a small subset of it (only 'reading'
statements, and no prepared statements), I also wrote a very simple
testing-tool which creates a new database with a table with 10 columns
of very different type, inserts some example data, and rereads that
data both using a simple Query() and using a prepared Statement().
This tool and a very short makefile to build it are also attached if
needed for the review-process. The tool uses the commands described
in the documentation of the abstract baseclass TSQLStatement to
simulate real-life behaviour.
Please note that tool is rather hacked-together without real error-handling,
as it is not meant for long-term usage, but might be helpful in the
review-process / adapted to a unit-test.

Apart from the mentioned specifics, nothing is really SQLite-only,
thus I did not change the documentation of the baseclass in the patch
(this documentation also lists all SQL-implementations). At least this
list in the documentation might be extended to make users aware of the
new implementation.