General polish

• Enabled smaller step intervals for filter values so that you can tune settings more precisely.
• Improved display of gamma value.
• Facilitated extension translation via https://lusito.github.io/web-ext-translator/?gh=https://github.com/PyvesB/night-video-tuner
• Optimised images so that the extension menu loads faster.
• Removed update notification so that the extension requires less permissions.
• Opera is no longer officially supported (see #2), but you can still install the extension via Install Chrome Extensions.
• Various other improvements and cleaning up.