style: include clearfix on blocks zebra rows & unify remove/zebra (via #45369)

wcs/qommon/static/css/dc2/admin.scss

@@ -3,6 +3,20 @@ $secondary-color: #00d6eb;
 $string-color: str-slice($primary-color + '', 2);
 $actions: add, duplicate, edit, remove;
 
+@mixin clearfix {
+	&::after {
+		display: block;
+		clear: both;
+		content: "";
+	}
+	@supports (display: flow-root) {
+		display: flow-root;
+		&::after {
+			content: none;
+		}
+	}
+}
+
 %block {
 	background: white;
 	padding: 0.5rem;
@@ -2115,6 +2129,7 @@ div.timetable-widget {
 .wcs-block-with-remove-button {
 	.BlockSubWidget {
 		position: relative;
+		@include clearfix();
 	}
 	.remove-button {
 		position: absolute;
@@ -2132,6 +2147,10 @@ div.timetable-widget {
 }
 
 .pk-blocks-zebra {
+	.BlockSubWidget {
+		position: relative;
+		@include clearfix();
+	}
 	// add a gray background for even numbered blocks, but that means odd
 	// childs when the first child is an error.
 	.error ~ .BlockSubWidget:nth-child(odd),