🦊 BREAKING: Use new XDG Base specification for state (#5)

Switched the history from using the XDG_CACHE_HOME directory by default
to use XDG_STATE_HOME by default.

This makes sense since cache can (and should be prepared to) be wiped at
any moment and the program functionality should not be hindered by this.
Since we need to retain history through such wipes the newly introduced
state directory is the perfect match for keeping the history file in.

This does constitute a breaking change for existing histories which need
to be moved to the new directory if they made use of the old cache
directory.

Concurrent with this we are renaming `XDG_CACHE_LOCATION` environment
variable to `XDG_HISTORY_LOCATION` so this is a second breaking change
for those using a custom location for their histories.

This change attempts to make the naming scheme coherent and remove some
left-over naming cruft from the old location being the cache directory.

This provides one of the larger changes to the program so far.

Fixes #5.

CHANGELOG.md

@@ -9,7 +9,14 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 <!-- ### Added -->
 
-<!-- ### Changed -->
+### Changed
+
+- (!) History uses `XDG_STATE_HOME` directory by default:
+  This constitutes a break in behavior if you relied a lot on your pick history in the default
+  location. To retain your old history file, simply move it from the old cache directory
+  (`~/.cache/bemoji-history.txt` by default) to the new one (`~/.local/state/bemoji-history.txt`
+  by default).
+- (!) `XDG_CACHE_LOCATION` renamed to `XDG_HISTORY_LOCATION` to better signify its purpose
 
 <!-- ### Deprecated -->