emojier: Show placeholder message instead of disabling recents when empty
Right now we disable the recents view when it's empty, but this relies on querying recentEmojiModel.count, which is not a property, so it doesn't update as the model fills up with recents. Making it a property is annoying, and a better UX is simply to show a placeholder message on the page when it's empty as we do in other places. The code already skips showing the view on launch if it's empty, so everything still works.
This also makes the placeholder menu appear when history is cleared, which is a nice bonus.
BUG: 443972 FIXED-IN: 5.24
