Blender Git Loki

UI: Confirm dialog when closing an unsaved file

The complexity in this patch comes from the fact
that the current operator system does not support
multi-step user interactions well.

More specifically, for this to work, we need to show
a confirm dialog and a file browser afterwards.
We decided that it is easier to keep everything in
a single operator, instead of creating separate
operators that invoke each other.

So, now the `WM_OT_open_mainfile` operator invokes
itself in different states. It implements a simple
finite state machine to manage the states.

The dialog itself is expected to be improved in
a future commit. See D4829 for more details.

Reviewers: brecht

Differential Revision: https://developer.blender.org/D4829