Minor configure.ac changes for eventual 3.0 release

[sedwards]

• Add message to end of configure script for next steps
• Switch default build from GTK+ 2.0 to GTK+ 3.0
• Mark GTK+ 3.0 support as no longer experimental
• Add option to test GTK 4.0 building as experimental
• Add a error message if all GTK libraries are disabled without passing --disable-gtkport
• Add a note that --disable-gtkport is experimental

The real main change here is switching the default from GTK+ 2.x to GTK+ 3.x line and marking the disable-gtkport option as experimental because it is currently broken anyway

[wdlkmpx]

Gtk3 is still experimental, there's an ugly hack to set a reasonable size for the log window, I applied something that works in my distro, but overall the sizes can slightly increase or decrease each time gftp is started. Stuff is broken. Overall the GtkPaned stuff looks really bad

I'll test this later, but I feel there's too much hope, gtk4 support will probably happen in 2040 and without my seal of approval

[sedwards]

I am not convinced gtk4.0 will ever happen, I just thought it would be good to have the configure option to try what we can. After thinking about, I agree that since the GtkTreeview stuff is not quite complete for the transfers, maybe it ought not be the default yet, maybe we should just leave this PR as a draft and then come back to it once you think Gtk3 is ready.

On a related note, I am going to close out my other PR about the XPM->PNG migration as if we are going to keep gtk2 around for a long time, we need to come up with a nice clean way to support all versions and image types so I will revisit that later.

[wdlkmpx]

Let keep this open for a while until the gtk3 ui is fixed, but I'm not sure what to do with the broken stuff. Gtk is good for simple dialogs and very primitive apps, anything else becomes a major PITA (and requires QT or something) as you port it to the next version that is more primitive and uglier