UI Improvements

• style/memorize_size: true/false can control if you want stable first candidate position, i.e. rime#523
• style/alpha: 0 is now valid, can thus will completely hide the panel
• Surrounding high lights for all candidates:
  • setting preset_color_schemes/xxx/candidate_back_color will enable them
  • style/surrounding_extra_expansion will control the relative size to the selected candidate's surrounding block. Can be negative.
• Various improvements:
  • style/border_height and style/border_width can now be negative, use in combination with style/corner_radius
  • style/line_spacing and spacing can now be negative.
  • All font_size can be float numbers.
  • labels are vertically centered when using a different label_font_size from the main font_size
• Allow simplifier to block comments when tips is set to none by adding allow_erase_comment: true under the simplifier's settings.