Persistent watch expressions (continuation of #150)

[Kevin-Prichard]

Summary

Persistence of watch expressions. This pull incorporates and continues with #150's fine work.

Details

• merged lechat:save_load_watches into inducer:main, and it works almost perfectly
• one tiny mod to var_view.py: to import pudb.debugger.CONFIG into local scope, to fix bug discovered after the merge
• another tiny mod to debugger.py: call save_watches() after a watch expression is deleted by the user, making that item become de-persisted from disk. This fixes an issue where the delete key tends to do nothing in the variables Watch expression subpane, following merge of lechat:save_load_watches into inducer:main.
• the issue occurs because load_watches() is called every time make_var_view() is invoked by update_var_view(), which itself is invoked frequently -whenever the UI needs redrawing. So, the removal of an expression by the delete key lasts for only approximately two shakes of a lamb's tail.

Tests

will add shortly