[PATCH] XDG base directory support (history and wxt)

[Jun. T <tak...@kb...>]

I'm not a fan of XDG basedir, but moving only ~/.gnuplot to XDG
directory may be a kind of "half done".

Attached is a possible patch for moving ~/{.gnuplot_history,.gnuplot-wxt}
to XDG directories (it seems qtterminal.conf is already taken care of).
A few points to discuss:

[1] Where to save the history?
There were three possibilities $XDG_{CONFIG,DATA,CACHE}_HOME, and probably
DATA directory is most widely used for history (but I'm not sure).
But to make the situation still more complicated, they added one more
directory $XDG_STATE_HOME recently. It seems this directory is most
suitable for history, but it is new and very few programs are using it yet,
and I'm rather suspicious of whether many programs that are already using
DATA (or CACHE?) directory for history will modify their code.
(and XDG may add more basedirs to make it further complicated...)

In the attached patch I use the STATE directory,
but I have no objection to using other directories.

Since only one file (the history file) is saved in the directory, I think
we can save it directly under XDG ( ~/.local/state/gnuplot_history)
instead of saving it as ~/.local/state/gnuplot/history.

[2] Migration
With the patch, gnuplot will continue to use traditional config or history
files in ~/ if they exist (as it does for the main config file ~/.gnuplot).
Users must move the traditional files to XDG dirs (or remove ~/.gnuplot-wxt
and ~/.gnuplot_history) manually if they want to migrate.

We could make gnuplot to force move traditional ones to XDG directories, but
I rather hesitate to do this.

[3] wxWidgets
They added a function (wxStandardPaths::FileLayout()) for supporting XDG
basedir in wxWidgets 3.1.1.
https://docs.wxwidgets.org/3.1.5/classwx_standard_paths.html
But even if FileLayout is set to FileLayout_XDG, it seems I still need to set
the full pathname of the config flie manually in wxFileConfig constructor to
make it saved in $XDG_CONFIG_HOME/gnuplot/ (i.e., not directly under
$XDG_CONFIG_HOME/). So I didn't use wxStandardPaths but used xdg_get_var() in
xdg.c and manually set the full pathname of the config file.

Good thing of this is it works with older wxWidgets (3.1.x is a kind of
development release).

I'm not familiar with wxWidgets. If there are anyone who knows wxWidgets better
please feel free to improve the patch.

[4] Documents
Are there any documents that need be updated?