wxGTK already did it like this, but also only declared ctor not taking
wxImage in this case, while wxMSW and wxMac declared this ctor in any
case, even when wxUSE_IMAGE==0, but didn't define it then.
This doesn't matter much anyhow, as the build with wxUSE_IMAGE==0 is
clearly broken and these changes are not enough to fix it, but be at
least somewhat consistent and:
1. Don't declare methods using a class which is not available at all.
2. Do define methods using wxImage only internally, even if they do
nothing when it's not available.
No real changes.
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