Fix #520 External plugin wrapper does not read background-image styles

Solution for #520 (closed)

Hopefully this is good - essentially rather than having a custom paint method for the external plugin container, just use CSS rules (like the panel itself does) and let GTK handle it. This allows the system theme to work correctly, and XFCE settings to override. Seems to work fine.

I use background CSS shorthand to make sure things like background-repeat and background-size etc. are default rather than system theme when overriding via XFCE settings.

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Also fixes: #161 (closed), #489 (closed)