Replace syllable overlay with native selection

[1ec5]

The syllable overlay is elegant but syncing it with the text will be a challenge, especially in non-left-aligned text or in situations where layout depends on the text length. Instead of managing an overlay ourselves, we could use nsISelectionController to set a special selection type, as I’m doing in #28. SELECTION_IME_RAWINPUT and SELECTION_IME_CONVERTEDTEXT look promising.