Allow plugins to handle Custom Draw in TreeView controls (bug #15077)

[oviradoi]

This PR attempts to fix bug #15077 .

I have modified the code that handles the CustomDraw messages sent by TreeView controls to also call the base subclass and merge the result with the one from the dark mode code.

This will allow the plugins to also handle the custom draw messages and add customizations to the TreeView items.

I have tested this with the Explorer plugin and also several built-in panels (Folder as Workspace, Document List).

[donho]

The eventual problem of this PR is: it could make the other plugins, which don't want to deal with the dark mode, broken.

@ozone10
What do you think about this PR?

[ozone10]

@donho
We already do this for listview. IMHO I would align the code with the listview’s implementation for consistency. However, I still find the current PR acceptable.

I would also include similar handling for toolbar in another PR, if requested.

[oviradoi]

I don't think it would break any plugin. If a plugin does not subclass a TreeView, the call to DefSubclassProc will not have any effect and the previous behavior will be kept. If a plugin does subclass TreeView, these changes will allow the plugin to add its own customizations.

[donho]

@oviradoi
Could you show me what's wrong of my screenshot:
#15077 (comment)

[donho]

@ozone10

We already do this for listview.

Could you point it out please?

[ozone10]

@donho
#11767

[donho]

@ozone10
Thank you!