[dragging] Trim the css property value (#1989)

Webview2 on Windows returns a potential whitespace when defining the
style like this `style="--wails-draggable: drag"`. Whereas Darwin already
trims the value and returns "drag" and not " drag".

v2/internal/frontend/runtime/desktop/main.js

@@ -87,7 +87,11 @@ window.addEventListener('mouseup', () => {
 });
 
 let dragTest = function (e) {
-    return window.getComputedStyle(e.target).getPropertyValue(window.wails.flags.cssDragProperty) === window.wails.flags.cssDragValue;
+    var val = window.getComputedStyle(e.target).getPropertyValue(window.wails.flags.cssDragProperty);
+    if (val) {
+      val = val.trim();
+    }
+    return val === window.wails.flags.cssDragValue;
 };
 
 window.wails.setCSSDragProperties = function (property, value) {