Previously the button was output as a part of the per-page option rendering, inline with that input. While this was appropriate for core's usage, the `screen_settings` filter has allowed plugins to place additional items at the bottom of the panel, which a number take advantage of. This leads to confusing situations where plugins that don't save settings via Ajax either have to add their own button or piggyback onto the existing button, which doesn't make any sense in the flow of additional options. It also hinders core from adding any other options that need to be submitted.
Also, when the screen options panel is open, a submit button there is the primary action at that moment. The "Apply" button also does a full page load, which a primary button indicates better.
fixes#34295. see #22222, #23738.
git-svn-id: https://develop.svn.wordpress.org/trunk@35161 602fd350-edb4-49c9-b593-d223f7449a82
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