Feature Request: Make text templates accessible from any number of document types

[noseshimself]

Right now the text snippets in the database table DocTexts are assigned to a single document type (and redundantly so by having the template number and the template name in the table -- one should suffice).

Certain phrases (e. g. in relation to due dates of invoices or delivery dates being equal to billing date) are useful across document types (e. g. all kinds of offers and invoices).

It might be a necessary addition to assign document types to groups (e. g. all invoice-items getting "invoice (general") as group type) to further improve this ability.

As Kraft is generally able to work with multiple languages it might be necessary to add a language specification to snippets. The consequences of that step will be the next feature request (#165).