Clear language data after opening file with a different extension

[RealPacket]

PR details

I'm surprised that no one wanted to fix it, although the only bug I'd say is that you would have open CTRL+L 2 times after opening a file with a different extension to get the new comments (after that, you don't have to do it until you open another file with a different extension, and so on). Fixes #81

Progress

• [X] Clear Keywords
• [X] Clear Color Regions
• [X] Clear Comments
• [X] Fix auto complete persisting from previous extensions.