move type AbstractBuilder.Context to new file to (hopefully) workaround an issue in scaladoc.

Currently 'play release' aborts with the following error:
...
[error] /home/uqageber/aorra/app/charts/builder/spreadsheet/JFreeBuilder.java:98: not found: type Context
[error]   protected Dataset getDataset(Context context) {
[error]                                ^
[error] 58 errors found
[error] (aorra/compile:doc) Scaladoc generation failed