Fix scale of Crop.amount to match object oriented model

Previously, we have been using 0 between 1 to denote that no crop is
growing, but now that we have moved the definition of amount to Crop
class member, we don't need the range for special meanings.
The more natural and intuitive definition of amount is to start off
at 0 and harvest at 1.
Change caption of seed and tuber buttons to product names

Namely, Seed becomes Corn and Tuber becomes Potato.
The problem is that Japanese translation of these words span longer
than English, making text go outside.  I have introduced font
customization feature for languages to shrink the size of the font.