Calculator: Add KeypadValue class

This commit adds a basic KeypadValue class which abstracts away
Keypad's internal representation in a slightly simpler format.
This will allow arithmetic operations in the Calculator to not
lose any precision. In cases where losing precision is necessary,
an explicit conversion operator to double is provided, as well as
an explicit constructor from double.

Userland/Applications/Calculator/CMakeLists.txt

@@ -10,6 +10,7 @@ set(SOURCES
     Calculator.cpp
     CalculatorWidget.cpp
     Keypad.cpp
+    KeypadValue.cpp
     CalculatorGML.h
 )