Fix a bug that caused QDecimalElement to ignore its keyboardType
property.

Moved keyboardType initialization to the constructor instead of
```getCellForTableView:controller```.

This allows users to override the keyboardType, which is useful on
iPad, since the default UIKeyboardTypeDecimalPad doesn't exist for
iPad, and UIKeyboardTypeNumbersAndPunctuation is a better alternative
than the standard keyboard.
Merge pull request #536 from bartvandendriessche/fix/allow_keyboardtype_overriding_for_qdecimalelement

Fix a bug that caused QDecimalElement to ignore its keyboardType