Improved system for installing things not available on the system when built.

usr/share/stormux/installable_packages.json

@@ -0,0 +1,14 @@
+{
+  "installable_packages": {
+    "thunderbird": {
+      "name": "Thunderbird Email Client",
+      "package": "thunderbird",
+      "verify_command": "command -v thunderbird"
+    },
+    "libreoffice": {
+      "name": "LibreOffice",
+      "package": "libreoffice-still",
+      "verify_command": "command -v libreoffice"
+    }
+  }
+}