Increase volume meter scale from -100 to 0 dB

This was done to enable users to to use higher precision values in the
"Audio" condition type.
Previously, the UI would reset the selected values to the minimum value
possible to display in the volume meter widget, which was -60 dB.

lib/utils/volume-control.cpp

@@ -619,7 +619,7 @@ VolumeMeter::VolumeMeter(QWidget *parent, obs_volmeter_t *obs_volmeter,
 	magnitudeColor.setRgb(0x00, 0x00, 0x00); // Black
 	majorTickColor.setRgb(0xff, 0xff, 0xff); // Black
 	minorTickColor.setRgb(0xcc, 0xcc, 0xcc); // Black
-	minimumLevel = -60.0;                    // -60 dB
+	minimumLevel = -100.0;                   // -100 dB
 	warningLevel = -20.0;                    // -20 dB
 	errorLevel = -9.0;                       //  -9 dB
 	clipLevel = -0.5;                        //  -0.5 dB