Markdown exits Edit mode and autosaves on lost focus

[gitterspec]

When using the Markdown widget in Edit mode, if the focus is lost even for a second (i.e. switching to a different window), Edit mode is exited and even worse, the behavior saves (not cancels) changes.

I think the behavior should keep it in Edit mode when switching focus. It's common to need to switch focus during editing, for example: copying something from another window for pasting.

The auto-save behavior is particularly dangerous, as the original note may be inadvertently lost by accident.