Fix compilation error due to std.xml deprecation

[cifvts]

std.xml has been deprecated [1] and removed [2] from the standard library, therefor compilation failed due to the missing package.

As a temporary fix, add the undeaD package and use the xml library from it for the time being.

Fix #2152

[1] https://dlang.org/changelog/2.092.0.html#deprecate-xml [2] https://dlang.org/changelog/2.101.0.html#remove-std-xml

[ximion]

This patch doesn't work for Meson (which in turn is what pretty much all Linux distros use to build Tilix), and relying on undeaD is, well, a dead end ;-) - and another dependency for people using Tilix to package.

I've updated the code to not rely on std.xml and use GMarkup instead. Thanks for the patch anyway, it is appreciated!

[cifvts]

Make sense, looking for another library was the next step, I will look what you have done, and use Meson myself, and see if the README could be updated too. Thanks