Add minus toggle for GuiValueBox & GuiValueBoxFloat (#485)

It was impossible to enter negative number.
Now pressing KEY_MINUS toggles sign for the value when value is not 0.
Due to original implementation it's not possible to start entry with minus symbol or have added minus while the value is 0.

fix max index

src/raygui.h

@@ -3045,6 +3045,34 @@ int GuiValueBox(Rectangle bounds, const char *text, int *value, int minValue, in
 
             int keyCount = (int)strlen(textValue);
 
+            // Add or remove minus symbol
+            if (IsKeyPressed(KEY_MINUS))
+            {
+                if (textValue[0] == '-')
+                {
+                    for(int i = 0 ; i < keyCount; i++ )
+                    {
+                        textValue[i] = textValue[i + 1];
+                    }
+                    keyCount--;
+                    valueHasChanged = true;
+                }
+                else if (keyCount < RAYGUI_VALUEBOX_MAX_CHARS -1){
+                    if (keyCount == 0){
+                        textValue[0] = '0';
+                        textValue[1] = '\0';
+                        keyCount++;
+                    }
+                    for(int i = keyCount ; i > -1; i-- )
+                    {
+                        textValue[i + 1] = textValue[i];
+                    }
+                    textValue[0] = '-';
+                    keyCount++;
+                    valueHasChanged = true;
+                }
+            }
+
             // Only allow keys in range [48..57]
             if (keyCount < RAYGUI_VALUEBOX_MAX_CHARS)
             {
@@ -3164,6 +3192,34 @@ int GuiValueBoxFloat(Rectangle bounds, const char *text, char *textValue, float
 
             int keyCount = (int)strlen(textValue);
 
+            // Add or remove minus symbol
+            if (IsKeyPressed(KEY_MINUS))
+            {
+                if (textValue[0] == '-')
+                {
+                for (int i = 0; i < keyCount; i++)
+                {
+                    textValue[i] = textValue[i + 1];
+                }
+                keyCount--;
+                valueHasChanged = true;
+                }
+                else if (keyCount < RAYGUI_VALUEBOX_MAX_CHARS - 1) {
+                if (keyCount == 0) {
+                    textValue[0] = '0';
+                    textValue[1] = '\0';
+                    keyCount++;
+                }
+                for (int i = keyCount; i > -1; i--)
+                {
+                    textValue[i + 1] = textValue[i];
+                }
+                textValue[0] = '-';
+                keyCount++;
+                valueHasChanged = true;
+                }
+            }
+
             // Only allow keys in range [48..57]
             if (keyCount < RAYGUI_VALUEBOX_MAX_CHARS)
             {