Resolve paths for default icon

[zappolowski]

This (partially) fixes #1173 - support for expanding variables is done in #1215

default_icon was erroneously declared as being a plain string while it can also be a full path. Thus changing its type should solve the described issue already (without any side effects I could currently see).

[fwsmit]

It seems a little weird to add custom code for expanding the $HOME environment variable. This might also behave differently than people expect. I think it's better to look at the wordexp() function to see if we can use this function for expanding like a shell would. Otherwise just having the ~ expansion seems better to me.

Adding a wordexp-based expansion might also give issues with characters not accepted by the shell, though. So that makes it a little more complicated. It might be cool for certain functionality.

[zappolowski]

I was on the edge whether I should expand $HOME at all ... but I'm also in favor to remove it again and just expand ~/ in the beginning of a string.

wordexp could also expand commands like default_icon = `rm -rf ~`/asdf/ if I'm not mistaken and thus I would either go for a white-list appraoch of environment variables to expand (like $HOME, $XDG_CONFIG_DIR, $XDG_DATA_DIR and alike) or just leave it as is.

[zappolowski]

I took a closer look at wordexp and it might be worth a try. As far as I can see, expansion of commands can be disabled and referencing a non-existing variable could be turned into an error - which I consider the best approach.

[zappolowski]

I've implemented a version of the resolution using wordexp. One thing to decide: shall we use WRDE_UNDEF or not?

1. If it is used, any undefined variable fails the expansion and the input is returned unaltered (with a warning).
2. If it is not used, it is replaced with an empty string, e.g. $XDG_CONFIG_HOME/dunst just becomes /dunst. But using it allows us to use special syntax like ${XDG_CONFIG_HOME:-$HOME/.config}/dunst.

[codecov-commenter]

Codecov Report

Merging #1210 (ca09495) into master (19865c0) will not change coverage.
The diff coverage is n/a.

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@@           Coverage Diff           @@
##           master    #1210   +/-   ##
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  Coverage   66.03%   66.03%           
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  Files          46       46           
  Lines        7595     7595           
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  Hits         5015     5015           
  Misses       2580     2580           

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[fwsmit]

I think it's better to keep the ~ expansion fix and the new feature of using wordexp to separate PR's. Could you create a new PR for the wordexp stuff? We can discuss it further there

[fwsmit]

Thanks, this makes it easy to review :)