Handle SDL_TEXTINPUT in TextEdit.

This is the only way to capture text in android. Together with showing the
virtual keyboard when pressing the menu button, this allows me to input text.
The unicode handling is still broken, though.
Use utf8ToWstr to support unicode again in text input.

I discovered this function which does exactly what I wanted. I don't think
using wstring internally makes much sense now, but for now it's ok.
Show vkeyboard on focus gain for text widget.

I don't think I need the auxiliary key to show the virtual keyboard. I can
automatically do that when entering input text mode. If I find any case I can
always readd it later.