[wip] history and sorting for dmenu mode

[awalgarg]

Alright, this is a veeeery rough patch, mostly intended to start discussion as discussed in irc.

Things we should probably do/change:

• The history setting should be done at once after stdout is completely flushed. This is for when in multi-select mode. Pretty small thing tbh, but maybe it matters.
• Not that great at C so dunno if code is idiomatic.
• Make it work with async mode.

[codecov-io]

Codecov Report

Merging #609 into master will decrease coverage by 0.02%.
The diff coverage is 64.7%.

@@            Coverage Diff             @@
##           master     #609      +/-   ##
==========================================
- Coverage   73.98%   73.95%   -0.03%     
==========================================
  Files          37       37              
  Lines        9163     9178      +15     
==========================================
+ Hits         6779     6788       +9     
- Misses       2384     2390       +6

Impacted Files	Coverage Δ
source/dialogs/dmenu.c	69.72% <64.7%> (-0.39%)	⬇️

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