Add an SQL Data Source

Add a possibility to enter a custom query as a data source.

added Improvement label

mentioned in issue #16 (closed)

mentioned in issue #41 (closed)

As workaround I used the sqltasks extension (https://github.com/systopia/de.systopia.sqltasks) from Systopia to create a view which then can be used as SQL source in Dataprocessor.

mentioned in issue #93 (closed)