filemanager: Add explicit context-menu button

src/theme.scss

@@ -1818,6 +1818,19 @@ tag-input {
         border: 2px solid transparent;
     }
 
+    .context-menu-action {
+        display: none;
+    }
+
+    tr:hover .context-menu-action {
+        display: inline-block;
+    }
+
+    tr.entry-selected .context-menu-action {
+        display: inline-block;
+        color: white;
+    }
+
     .entry-hovered {
         border: 2px solid $brand-primary;
     }