Improve performance of the emoji model

Only display up to 10 emojis when searching

Related to #310 but a better solution needs to be found because we get
the same problem on big room when autocompleting usernames.

imports/NeoChat/Dialog/EmojiDialog.qml

@@ -25,6 +25,5 @@ QQC2.Popup {
 
     contentItem: EmojiPicker {
         onChosen: react(emoji)
-        emojiModel: EmojiModel {}
     }
 }