Lv2: Improve LED widget's digit calculation

This improves the way digits are calculated for display in
`Lv2ViewProc`:

- More than 2 digits are now recognized
- Minus signs are now recognized
- Tests have been added

include/lmms_math.h

@@ -325,6 +325,32 @@ static inline T absMin( T a, T b )
 	return std::abs(a) < std::abs(b) ? a : b;
 }
 
+// @brief Calculate number of digits which LcdSpinBox would show for a given number
+// @note Once we upgrade to C++20, we could probably use std::formatted_size
+static inline int numDigitsAsInt(float f)
+{
+	// use rounding:
+	// LcdSpinBox sometimes uses roundf(), sometimes cast rounding
+	// we use rounding to be on the "safe side"
+	const float rounded = roundf(f);
+	int asInt = static_cast<int>(rounded);
+	int digits = 1; // always at least 1
+	if(asInt < 0)
+	{
+		++digits;
+		asInt = -asInt;
+	}
+	// "asInt" is positive from now
+	int32_t power = 1;
+	for(int32_t i = 1; i<10; ++i)
+	{
+		power *= 10;
+		if(static_cast<int32_t>(asInt) >= power) { ++digits; } // 2 digits for >=10, 3 for >=100
+		else { break; }
+	}
+	return digits;
+}
+
 
 } // namespace lmms