Progress Bar and New Gestures

New Features：

• a progress bar is added on the top of the quiz page for tracking quiz process in real time
• add new gestures (keyboard shortcuts):
  • Ctrl + N in Word/Tag Editor: clear the board for a new word,
  • Ctrl + Shift + C in Word Editor: clear the word spelling and definition but preserve other inputs on the board, preparing for a new word from the same context, tags, etc.

Bugfixes：

• fixed the problem that erroneously index the records when loading a new quiz after previous quizzing.

Others：

• Change the gesture of "Create Word/Tag" back as Ctrl + Shift + Enter to avoid the conflit with adding newline in textbox.