IOM: Disable "Remove" menu item when no transformer is selected (easy)

[Zardshard]

The Transformer->Remove menu item should ideally be disabled when no transformer is selected, similar to how Shape->Remove, Style->Remove, and Path->Remove function when no shape/style/path is selected.

Note: The Transformer menu will be disabled until you create a shape. Try Shape->Add empty, then you should be able to access the Transformer menu.

Since I'm marking this as easy, here's a hint to get you started: StyleListView::_UpdateMenu() is where the style list enables/disables the Style->Remove menu item. You'll need to update TransformerListView::_UpdateMenu() to do something similar.

This option was introduced in hrev58502 (​https://cgit.haiku-os.org/haiku/commit/?id=3aa371a2852fe031e6aa4cdf7db1cb56c931b9bb)