drop-overlay: Introduce GcalDropOverlay

A new widget to perform drag and drop operations, for example,
dropping ICS files to the GcalWindow. It allows to overlay
a child for visual feedback when a DnD operation is done.

either in the main window or in the import dialog,
depending on whether none could be imported, or just some
of them.

It is based on Loupe's DragOverlay widget.

Closes #1257

src/theme/style.css

@@ -483,3 +483,24 @@ menubutton stack > box {
 syncindicator {
   padding: 6px;
 }
+
+/*
+ * Styles for the drop overlay.
+ * From Amberol https://gitlab.gnome.org/World/amberol/-/blob/0dd6250ffd294b14532c428dcb76ae42086a5923/src/gtk/style.css
+ */
+
+/* Removes the default style */
+.drop-target:drop(active) {
+  box-shadow: none;
+}
+
+/* Custom style for the drop overlay area */
+.drop-overlay-area {
+  padding: 32px;
+  color: var(--accent-fg-color);
+  background-color: alpha(var(--accent-bg-color), 0.5);
+}
+
+.blurred {
+  filter: blur(6px);
+}