[FIX] Fix for Issue 2716: Hide 'Print' button for the report 'Timesheet Profit'

The report 'Profit Timesheet' shouldn't be directly displayed in the views related to the model 'account.analytic.line' but only from the reporting menu. In facts the report requires a popup to specify the report parameters.

Fixes #2716
[IMP] fields: reduce the number of attributes on _column instances

This is a memory optimization: instead of setting all attributes on all
instances, set them with their default value on the class, and only set
specific ones on instances.  This reduces the memory footprint of around 14Mb
per registry with modules crm, sale, purchase and stock installed.
[FIX] fields: inherited fields should get 'string' from their parent field