Fix qdarkstyle and midnight blue sliders being cropped

- Allow Threshold sliders in control configuration to be up to 40px tall, instead of 15

dist/qt_themes/qdarkstyle/style.qss

@@ -944,6 +944,10 @@ QListView::indicator:indeterminate:pressed {
     image: url(:/qss_icons/rc/checkbox_indeterminate_focus.png);
 }
 
+QSlider:horizontal {
+    height: 20px;
+}
+
 QSlider::groove:horizontal {
     border: 1px solid #565a5e;
     height: 4px;

dist/qt_themes/qdarkstyle_midnight_blue/style.qss

@@ -1296,6 +1296,10 @@ QSlider:focus {
   border: none;
 }
 
+QSlider:horizontal {
+  height: 20px;
+}
+
 QSlider::groove:horizontal {
   background: #32414B;
   border: 1px solid #32414B;