Add icon-static server capability

[hastinbe]

Dunst supports displaying the first frame of an icon, thus it seems reasonable that it should announce that capability with icon-static.

"icon-static" | Supports display of exactly 1 frame of any given image array. This value is mutually exclusive with "icon-multi", it is a protocol error for the server to specify both.
from https://developer.gnome.org/notification-spec/#hints

I check for this capability in another project. notify-osd is the only other I know that supports/announces it, however dunst doesn't when it could since it has this already. I've been relying upon the image-path hint as an alternative which is more or less a fire and forget approach but it'd be nice to know if the capability exists

[codecov-commenter]

Codecov Report

Merging #867 (bd6abfc) into master (f2017eb) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #867      +/-   ##
==========================================
+ Coverage   58.54%   58.55%   +0.01%     
==========================================
  Files          37       37              
  Lines        5987     5989       +2     
==========================================
+ Hits         3505     3507       +2     
  Misses       2482     2482              

Flag	Coverage Δ
unittests	58.55% <100.00%> (+0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
src/dbus.c	63.14% <100.00%> (+0.11%)	⬆️
test/dbus.c	98.67% <100.00%> (+<0.01%)	⬆️

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[fwsmit]

I didn't know we supported this :-)
LGTM, thank you.