Add option to auto-clear on cold and warm app starts

[PhrozenByte]

First of all: Great app, thank you ❤️

Currently Calculator++ (as of version 2.3.3) always starts with the previous input and calculation result, requiring one to manually clear (C) the calculator before entering a new formula. This annoys me a little.

Thus I want to suggest adding an option to auto-clear the previous input when starting the app. This should be limited to cold and warm app starts, i.e. not include hot starts (i.e. when Calculator++ is still in memory, it shouldn't auto-clear, otherwise it should). If one needs to access the previous calculation, one can use the history feature. The default behavior could remain unchanged (i.e. opt-in).