BlueprintForm: Make `import@`s to support ordering

[mahagr]

Right now all calls for @import insert the new fields into the top of the form.

Figure out a way to be able to insert fields also to the end of the form or between existing fields.

[drzraf]

PR #34 intends to resolve this.
The second commit allows top-level ordering@: 0 to put an element as the first one, fixing #33