Add popup of recent documents to "Open" action toolbar button

This changes the "Open" action to a `KToolBarPopupAction` where, when
placed in a toolbar, it will turn into a split button with recent documents
listed from the accompanying file_open_recent action.

This works automatically for all regular KXmlGui applications that have a
"Recent Documents" action alongside their "Open" action, such as Kate,
KWrite, Kolourpaint, etc.