Fixed Folder as Workspace toolbar button not activated issue

Dropping folder into Notepad++ to have Folder as Workspace not make its
toolbar button activated. This bug is fixed by this commit.

Fixes #3282, closes #3501

PowerEditor/src/Notepad_plus.cpp

@@ -5767,6 +5767,10 @@ void Notepad_plus::launchFileBrowser(const vector<generic_string> & folders)
 	}
 
 	_pFileBrowser->display();
+
+	checkMenuItem(IDM_VIEW_FILEBROWSER, true);
+	_toolBar.setCheck(IDM_VIEW_FILEBROWSER, true);
+	_pFileBrowser->setClosed(false);
 }