Calculator: Add a Rounding menu

This menu has three actions. Each one of them enable a different level
of rounding: to 0, 2 or 4 digits.

Userland/Applications/Calculator/Keypad.h

@@ -29,6 +29,8 @@ public:
     void set_value(Crypto::BigFraction);
     void set_to_0();
 
+    void set_rounding_length(unsigned);
+
     String to_string() const;
 
 private:
@@ -46,6 +48,8 @@ private:
 
     mutable Crypto::BigFraction m_internal_value {};
 
+    unsigned m_displayed_fraction_length { 0 };
+
     enum class State {
         External,
         TypingInteger,