Add option to force keyboard into incognito mode

[BloodRaven0]

Given that the diary is supposed to be private, and you added so many privacy options, I think this one is a must.

Most keyboards, like Swiftkey and Gboard save user-written words and how they appear in a sentence to better predict words. However, the word prediction sometimes can recreate entire sentences that you've once written somewhere if you start with the right word.

That is why these keyboards have "incognito" modes that turn off the word learning algorithm, and it can be turned on automatically by the app you're using, for example when you're browsing in incognito mode in your browser.

I think a toggle to turn on this mode while in the app will be really useful, because some people use it for really sensitive topics and wouldn't want their diaries suggested during a chat with a friend or, even worse, when they give their phone to someone.