LibGUI: Add a simple emoji input dialog activated by Ctrl+Alt+Space :^)

Widgets can now opt in to emoji input via set_accepts_emoji_input().
If the focused widget accepts emoji input, we'll pop up a simple dialog
with all the available emojis as clickable buttons.

You can press escape if you change your mind and don't want an emoji.

This UI layout definitely will not scale as we add more emojis, but it
works for the moment, and we can adapt it as we go. Pretty cool! :^)

Libraries/LibGUI/CMakeLists.txt

@@ -21,6 +21,7 @@ set(SOURCES
     Dialog.cpp
     DisplayLink.cpp
     DragOperation.cpp
+    EmojiInputDialog.cpp
     Event.cpp
     FilePicker.cpp
     FileSystemModel.cpp