ViewPropertiesDialog: disable Apply button on startup

We call `markAsDirty(false)` at the end of `loadSettings()` but that's
not enough, because `m_isDirty` is initialized to false (as it should be).
This means `isDirtyChanged` is not emitted and the button is not disabled.

By disabling it explicitly, we make it consistent with the Apply button
in the settings dialog (which is also initially disabled).