fix: add missing css rule to hierarchy panel

[jperedadnr]

Issue

Fixes #773

The hierarchy panel (a StackPane) needs to set the padding to 0 as well, otherwise, when using Scene Builder Kit from an external application, modena default padding:

.titled-pane > .content > StackPane,
...
.titled-pane > .content > VBox {
    -fx-padding: 0.8em;
}

is set for the pane.

Progress

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