Add changelog for version 1.2.3

Author: frou

docs/packagecontrol_changelogs/1.2.3.txt

@@ -0,0 +1,6 @@
+-> 1.2.3
+
+FIXED:
+
+- Fixed [Prev] [Copy] [Show] buttons not appearing in the new Sublime Text beta.
+  Credit: jtojnar https://github.com/frou/st3-gitblame/pull/56

messages.json

@@ -1,5 +1,6 @@
 {
   "1.2.0": "docs/packagecontrol_changelogs/1.2.0.txt",
   "1.2.1": "docs/packagecontrol_changelogs/1.2.1.txt",
-  "1.2.2": "docs/packagecontrol_changelogs/1.2.2.txt"
+  "1.2.2": "docs/packagecontrol_changelogs/1.2.2.txt",
+  "1.2.3": "docs/packagecontrol_changelogs/1.2.3.txt"
 }