feat: Steamify's choices in the installer: a Steam Machine page (what to leave out) and a Start in page (gaming mode or desktop), passed to steamify.sh --defaults

archiso/airootfs/etc/calamares/modules/packagechooser_steamifyboot.conf

@@ -0,0 +1,16 @@
+# Steamify: where the Steam Machine starts (steamify-install passes --boot).
+---
+mode: required
+method: legacy
+labels:
+    step: "Start in"
+default: gamescope
+items:
+    - id: gamescope
+      name: "Gaming mode"
+      description: "Start straight into Steam's gaming mode, like SteamOS. Switch to Desktop in Steam's power menu brings you to the desktop."
+      screenshot: ":/images/no-selection.png"
+    - id: desktop
+      name: "Desktop"
+      description: "Start in the Plasma desktop. Return to Gaming Mode on the desktop brings you to Steam's gaming mode."
+      screenshot: ":/images/no-selection.png"