Cms: Add option to make default primary save action "save as draft"

[groe]

(Follow-Up to #911)

Description

For my client it would be useful to replace the "Save" button with "Save as a draft" button. Saving a draft is already possible via the save buttons dropdown menu, but they would like to prevent any accidental publishing of changes to existing pages as well, not only new ones.

So it would be great if there was a config setting to allow setting the default saveEntryAction to either saveEntry or saveEntryDraft, or something like that.

Steps to reproduce

1. Edit an existing entry (or create a new one)
2. Click save button
3. Changes are now published, and not only saved as a draft

[FrisovanDijk]

I would like to add that the expected default behaviour when clicking 'Save' is _save and continue editing_. Right now it's _save and close_.

Being able to set the default behaviour is a good option to avoid such issues.