Clear button "C" is changed to "AC", even if there wasn't calculation

[Bartosz Kosiorek]

Clear button "C" is changed to "AC", even if there wasn't calculation.

This behavior causing a lot of issues during implementation, and it is error prone.

Steps to reproduce:
1. Type number "100"
2. Press "="

Result:
- No calculation made. Button changed to "AC"

Expect:
- No calculation made. Button still "C"