[FIX] ir_qweb: allow to pass context to the ir_ui_view loader

When ir_qweb wants to load a new template, it uses the loader method received
as an argument. This method was always using the original context, so the
t-context feature was not working. Now, if a context is defined, it uses this
one and completely ignore the original.
Also, the merged context was not assigned to the right variable.