Use SDL_StartTextInput for gold drop dialog

[obligaron]

Fixes #444

Notes:

• Should work like previous input method but based on SDL_TEXTINPUT/SDL_StartTextInput.
• Tested with SDL2 and Windows only. So please retest with controller and SDL1.
• Converting from char to digit is based on 0-char. So it incorrectly recognize a unicode sequence as digit. But the same could is true for old input method and this is only relevant if you try to enter invalid input. This could be investigated in a follow up pr later.

[AJenbo]

Excellent, works flawlessly 👍 This means that there are no strictly broken features when using touch, only some minor quirks 🎉

retest with controller

Not relevant, a bare controller has no way to input text as things are. But it might happen one day: #2249

Converting from char to digit is based on 0-char.

Out of curiosity, why not atoi()?

[obligaron]

Out of curiosity, why not atoi()?

atoi() returns 0 for invalid input. But we want to distinguish between invalid input and 0 char.
An alternative could be strtol.
The old implementation uses this approach and I didn't want to experiment too much before 1.3 is released. 🙂