Calculator: Add a "Custom" entry to the rounding menu

This entry pop a dialog to ask the user to enter a value. The Calculator
will automatically put itself in this mode if you enter a number with
more digits in the fractional part than the actual maximum length.

Userland/Applications/Calculator/CMakeLists.txt

@@ -9,6 +9,7 @@ set(SOURCES
     main.cpp
     Calculator.cpp
     CalculatorWidget.cpp
+    RoundingDialog.cpp
     Keypad.cpp
     CalculatorGML.h
 )