Use the DPI setting within the i3bar (#2556)

libi3/dpi.c

@@ -64,6 +64,14 @@ init_dpi_end:
     }
 }
 
+/*
+ * This function returns the value of the DPI setting.
+ *
+ */
+long get_dpi_value(void) {
+    return dpi;
+}
+
 /*
  * Convert a logical amount of pixels (e.g. 2 pixels on a “standard” 96 DPI
  * screen) to a corresponding amount of physical pixels on a standard or retina