feat(audio): remove custom notification sounds feature

The main purpose of this project is to optimize vk.com (even through
extension), but not to provide such customization features.
Only really useful things.

So remove custom notification sound themes completely.

This closes #145.
chore(init): remove `vkCheckLoadedScripts()` function

Remove obsolete unused `vkCheckLoadedScripts` function from `vkopt.js`.

This closes #149.
fix(audio): make titles fixed-width'ed in inline player

Make the width of audio titles fixed (290 px), so that the size element,
which is a child of the title, is always aligned to the right.

This closes #142.
refactor(meta): use "vk.com" instead of "VK.com"

Replace "VK.com" with "vk.com" everywhere.
Why? It just looks nicer to me.

This closes #150.
fix(maxthon meta): save def.json with encoding UTF-8 with BOM

Maxthon Extension Center requires manifest to be in UTF-8 with BOM.

This closes #152.