Always enable midline color, never None (#291)

Previously, midline_color was None by default but #404040 in the template.
Now it's #404040 by default, and changed from Optional[str] to str.

Eliminates bug where unchecking "midline color" (midline_color=None)
switches to blue, instead of transparent.

corrscope/corrscope.py

@@ -122,12 +122,7 @@ def default_config(**kwargs) -> Config:
         channels=[],
         layout=LayoutConfig(orientation="v", ncols=1),
         render=RendererConfig(
-            1280,
-            720,
-            res_divisor=4 / 3,
-            midline_color="#404040",
-            v_midline=True,
-            h_midline=True,
+            1280, 720, res_divisor=4 / 3, v_midline=True, h_midline=True
         ),
     )
     return attr.evolve(cfg, **kwargs)

corrscope/gui/view_mainwindow.py

@@ -173,7 +173,7 @@ class MainWindow(QWidget):
                 with add_row(s, "", OptionalColorWidget) as self.render__grid_color:
                     pass
 
-                with add_row(s, "", OptionalColorWidget) as self.render__midline_color:
+                with add_row(s, "", BoundColorWidget) as self.render__midline_color:
                     pass
 
                 with add_row(s, BoundCheckBox, BoundCheckBox) as (

corrscope/renderer.py

@@ -154,7 +154,7 @@ class RendererConfig(
     grid_color: Optional[str] = None
     stereo_grid_opacity: float = 0.5
 
-    midline_color: Optional[str] = None
+    midline_color: str = "#404040"
     v_midline: bool = False
     h_midline: bool = False