Add Meson port with layer-shell support

[dimkr]

This is pretty much a 1:1 port of layer-shell support in https://github.com/puppylinux-woof-CE/gtkdialog (mostly @01micko's work) to yad.

Supporting the layer-shell protocol makes it possible to use yad for things like splash screens, notifications, panels and all kinds of floating widgets, when running under a wlroots-based compositor. The first use case I see is replacing yaf-splash and gtkdialog-splash with something simpler that uses yad under the hood, because yad is more maintainable and more likely to be ported to GTK+ 4 and beyond.

To make life easier and maintain backward compatibility, this PR adds basic Meson build system support (without optional features like the icon browser) that builds the GTK+ 3 variant and auto-enables layer-shell support if possible.

[step-]

Thank you Dima, it looks good. Could you please possibly replace strcmp with strcasecmp for uniformity with other option setting code in the file, and also add a space between function names and the opening bracket? I would do it myself but then I'm not sure if GH would give you attribution for this PR.

[dimkr]

replace strcmp with strcasecmp

Done

add a space between function names and the opening bracket

Done

[step-]

Thank you again!

[dimkr]

Thank you @step-! I didn't expect this level of cooperation and already accepted the fate of patching my distro's yad forever. Maybe there will be more to come, once I finish porting all gtkdialog-based stuff to yad and find more use cases yad doesn't address.