Move to chromium/electron-native spellchecker

[vladimiry]

Since version 8 @electron comes with chromium-builtin spellchecker functionality electron/electron#20692 Stable version 8 is not released yet, but anyway I think I better start experimenting after version 9+ goes live, so the functionality gets stabilized as spellchecker stuff is not so trivial when it comes to multi-OS support.

[vladimiry]

I was observing the chromium-builtin/native spellchecker related issues in https://github.com/electron/electron/issues for more than a year and what I noticed is that:

• the long-standing issues are still there (like for example downloading the dictionaries from google CDN even though the feature was not explicitly enabled)
• new valid issues get placed (like it worked before but then stopped working in new @electron versions)

So I think the time for switching to chromium-builtin/native spellchecker has not come yet.

[vladimiry]

Closing if favor of #469.