NEW FEATURES like percent

Meanwhile, I've added three new buttons:
1. DeleteOne: Now you can erase one number of the calculation.
2. Percent: If type a number in and press the percent button, it gives you the number divided by 100.
3. Modulo: Execute a modulo operation.

Thank you!

Main math operators and delete button

NEW:

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Finally: you can use this calculator like a real calculator. I've added the main mathematical operators, so you can make use of the fancy calculator.

Additionally, I've been created a delete button, thus you're now able to delete the whole calculation if you have mistyped.

I welcome any criticism, feedback or suggestions, so feel free to write me.

And now enjoy yourself with the breathtaking calculator.

And thanks for the partial help @Fornball.