Replaced deprecated NPPM_ADDTOOLBARICON with NPPM_ADDTOOLBARICON_FORDARKMODE to support dark mode icons

Author: Fruchtzwerg94

Visual Studio Project Template C#/PluginInfrastructure/NotepadPPGateway.cs

@@ -41,7 +41,7 @@ public void AddToolbarIcon(int funcItemsIndex, toolbarIcons icon)
 				Marshal.StructureToPtr(icon, pTbIcons, false);
 				_ = Win32.SendMessage(
 					PluginBase.nppData._nppHandle,
-					(uint) NppMsg.NPPM_ADDTOOLBARICON,
+                    (uint) NppMsg.NPPM_ADDTOOLBARICON_FORDARKMODE,
 					PluginBase._funcItems.Items[funcItemsIndex]._cmdID,
 					pTbIcons);
 			} finally {