Grid view support multi-select by drag (#6420)

* grid view support multiple selection by drag

* fix conflict with multiple selection by keyboard

* improve selection experience and code format

* fix bug - select failed when file name include '...'

frontend/src/css/grid-view.css

@@ -85,3 +85,9 @@
 .grid-drop-show {
   background: #f8f8f8;
 }
+
+.selection-box {
+  position: absolute;
+  background-color: rgba(0, 120, 215, 0.3);
+  border: 1px solid rgba(0, 120, 215, 0.8);
+}

frontend/src/css/lib-content-view.css

@@ -56,6 +56,7 @@
 }
 
 .dir-content-main {
+  position: relative;
   flex: 1 0 74.5%;
   display: flex;
   flex-direction: column;