[ADD] base: report: qweb-text type

This commit adds a new ir.action.report type: qweb-text. It produces text files,
serves them with a text mimetype and these reports use the same rendering context
as the current other types (PDF and HTML).

This will be used in a following commit to produce ZPL scripts. Those scripts
are used for printing thermal label on Zebra printers.

task ID : 1837175