Minor fixes and improvements.

1. add copy-to-clipboard function.
2. bug fix on exponent number display by using a fixed width digital font.
3. add comma as keyboard shortcut when using comma as decimal point.
4. optimize keyboard shortcuts to use one character in most cases.

bug fix.

We have improved it a lot, please check them out:
1. add keyboard shortcuts for all keys.
2. add more preference settings: key click feedback, using comma as decimal point, and so on.
3. add calculator quick start help, keyboard shortcuts mapping help, user guides, and online support access in help menu.

bug fix for some minor issues.

add support to save and restore settings.