Adds a preference and an project preference page

The preference is initialized with framework defaults (play.api.templates._, play.api.templates.PlayMagic._).
It can be overriden at the project level, in the project properties.
Changes are not applied directly, as we are currently lacking a real builder.

Fix #35.
Merge pull request #43 from skyluc/issue/additional-imports-35

Adds a preference and an project preference page