[GTK] Transparent combo popups

[akurtakov]

Combo boxes popup is transparent when PARENT_BACKGROUND is set as Control's implementation relies on transparency to implement it on GTK.
One instance of the problem has been eclipse-pde/eclipse.pde#2094 .
Other projects put workaround for it like eclipse-swtchart/swtchart#347 and eclipse-chemclipse/chemclipse#1372

[laeubi]

@akurtakov the same happen in the E4 Model editor, very annoying!

[vogella]

Also visible in the Combo of Eclipse Copilot plug-in