OSK: Add a button to let the user enter spaces.

Square will serve as the spacebar, and based on the language, will input an appropriate space (e.g. full-width space for Japanese or English full-width, regular space for everything else).

Fixes/implements https://github.com/hrydgard/ppsspp/issues/5634.

Here's how the OSK looks now, with 4 rows of characters (all languages except for Korean):
![Screenshot 01](http://i.minus.com/i9ODNzm4tPpNV.jpg)

With 5 rows of characters:
![Screenshot 02](http://i.minus.com/ibo5h9hv4F6qQ4.jpg)