Improve Terminal appearance and add Droid Sans Mono as default font

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Droid font is an "older package" in Ubuntu based on Linux Lite's adoption:
http://packages.ubuntu.com/ja/trusty/fonts-droid
http://packages.ubuntu.com/ja/xenial/fon...d-fallback

It was temporarily deleted, but it is left as a display support for some flavors.
Instead of switching to Google Noto Fonts. Please be aware of this point.