Gtk3 migration - get rid of many pixmaps and lists

This is the next step in migration to gtk3 - getting rid of many pixmap usages as well as lists, as those are not available in gtk3 any more. I also fixed the issue of pop3 authentication not working (not sure if this is the same issue as smtp in #39). Very odd - with the previous code this should have never worked. Pop3 sending emails is now working. What I must apologise for is that in previous commit for compiler warnings, I had set some brackets wrong that broke 16bit icon generation for batch dialog, this is now fixed. The other thing to apologise for is that I did some further testing of comboboxes with the test xsane backends, and there were two further cases where the comboboxes got destroyed when not protected with an additional g_object_ref. I must really do some debugging on this (maybe it is possible to backtrace at what point of the program the box gets deleted, at the moment I only see the point when it already has been deleted and its usage fails), but I don't understand at the moment why this is happening. After all, we just create a replacement widget and it is packed within a parent, so it should be protected against removal. Anywhere, there are now 3 instances where I additionally reference an object for the moment.

This should be all replacements of abandoned widgets in gtk3 (apart from the two option menus that have context menus) - the next thing will have to be some cairo graphics.