[IMP] tree_but_open - persist values in context

On action "tree_but_open" (double-click on a row in the list or tree)
current behavior is to throw out all values of old context. this patch
makes it possible to persist some values of context by putting a list of
keys to be persited with a key "persist_values" in the context itself.
Thereby it solves the problem described in the following question:
https://www.odoo.com/forum/help-1/question/how-to-preserve-context-across-multiple-actions-wizards-64377
-in addition it may introduce new possibilities in usage of tree_but_open.
This changes are very safe and they don't affect any existing functionality,
please merge it into the main branch.

 On branch 7.0
 Changes to be committed:

	modified:   addons/web/controllers/main.py