Introduce a context manager that temporarily overrides a UILayout's
operator context.
Instead of writing this:
```py
default_op_context = layout.context
layout.context = 'OTHER_VALUE'
layout.do_stuff()
layout.context = default_op_context
```
you can now write this:
```py
from bl_ui_utils.layout import operator_context
with operator_context(layout, 'OTHER_VALUE'):
layout.do_stuff()
```
This is also exception-safe; it will always ensure the layout's operator
context is restored when the `with` body is exited, regardless of
whether that's done with an exception or regularly.
Idea in-the-hallway approved by @Sergey.
