New engine: fewer wrong corrections, smarter suggestions

A batch of changes to the new typing engine, mostly about fewer wrong corrections and a suggestion bar you can trust more.

• Autocorrect holds back when it isn't sure. If a correction only barely beats the word you typed, your word stays and the fix waits in the bar. In my tests this cut wrong corrections by about a fifth.
• Split words get joined: "keyb oard " becomes "keyboard ". Backspace undoes it, and a pair you undid won't be joined again.
• Missing spaces get fixed when the split is clearly right: "thisis" becomes "this is".
• In Spanish, ¿ and ¡ start a new sentence, so the word after them is predicted like the first word of a sentence.
• Predictions are faster and a bit more accurate, and words you taught the keyboard now show up in them. For words you use often that saved about 3 % more keystrokes in my tests.
• When you pick a correction from the bar, the keyboard remembers the pair and is more likely to make that fix next time. Undoing it forgets the pair.
• The middle suggestion is bold only when the keyboard is fairly sure, at least twice as likely as the next one. Before, it was always bold.
• The keyboard learns where you tap. If you keep hitting a bit left of the keys, it starts to compensate after about 30 letters of correctly typed words and is fully adjusted after about 100. The shift is capped, and it never learns in password fields or fields without autocorrect.
• Joining split words no longer slows down typing. It runs in the background and never touches text you've already typed past.
• "fox" no longer turns into "Fox" (same for frank, creek, avenue and others). This one needs the updated English dictionary, which comes as a separate download.