Add currently-written-letters indicator to japanese text when using qml textedit

When writing a Japanese word, the currently written letters should be highlighted by an underlying line. This line indicates all the current letters that have been written until the word is "accepted". When the word is accepted, the line disappears and the written letters become background-highlighted for choosing a word. After choosing the word, this background highlight also disappears and user can continue writing text again with the underlined letters.

Qt Widgets TextEdit already behave in this way, the underline behaviour should be copied over from there. The background selection mode is already present and working as intended.

Steps to reproduce this behaviour:

• Switch to Japanese IME
• Write some Japanese letters, notice that these letters should be highlighted by the underlying line
• Press spacebar, the highlighted letters become "selected", and you can choose a word that they convert to (if you press spacebar again, it will popup the word list). You can also press enter directly to only select the letters you wrote without converting to a word
• Press enter to select a word, all highlighting and selections disappear
• Repeat writing a word, the letters are highlighted with underlying line again