overlay: minor UI fixes for clear all / reset all (#580)

## Link to GitHub Issue or related Pull Request, if one exists
#578 

## Description of change
Use `Clear All` when there are no defaults, and `Reset All` when there
are defaults.

Clearing analogs should reset values to 0.5, not 0.

src/spice2x/cfg/analog.h

@@ -91,6 +91,7 @@ public:
         setMultiplier(1);
         setRelativeMode(false);
         setDelayBufferDepth(0);
+        setLastState(0.5f);
     }
 
     inline void clearBindings() {