Move printer demo and energy-monitor into new top-level demos/ folder

These are showing off use-cases for Slint, but they're not examples showing individual Slint features.

Also removed the old printerdemo while at it.

demos/printerdemo/python/main.py

@@ -0,0 +1,65 @@
+# Copyright © SixtyFPS GmbH <info@slint.dev>
+# SPDX-License-Identifier: MIT
+
+from slint import Color, ListModel, Timer, TimerMode
+import slint
+from datetime import timedelta, datetime
+import os
+import copy
+import sys
+sys.path.append(os.path.join(os.path.dirname(__file__), ".."))
+
+PrinterQueueItem = slint.loader.ui.printerdemo.PrinterQueueItem
+
+
+class MainWindow(slint.loader.ui.printerdemo.MainWindow):
+    def __init__(self):
+        super().__init__()
+        self.ink_levels = ListModel([
+            {"color": Color("#0ff"), "level": 0.4},
+            {"color": Color("#ff0"), "level": 0.2},
+            {"color": Color("#f0f"), "level": 0.5},
+            {"color": Color("#000"), "level": 0.8},
+        ])
+        # Copy the read-only mock data from the UI into a mutable ListModel
+        self.printer_queue = ListModel(self.PrinterQueue.printer_queue)
+        self.PrinterQueue.printer_queue = self.printer_queue
+        self.print_progress_timer = Timer()
+        self.print_progress_timer.start(
+            TimerMode.Repeated, timedelta(seconds=1), self.update_jobs)
+
+    @slint.callback
+    def quit(self):
+        self.hide()
+
+    @slint.callback(global_name="PrinterQueue", name="start_job")
+    def push_job(self, title):
+        self.printer_queue.append(PrinterQueueItem(
+            status="waiting",
+            progress=0,
+            title=title,
+            owner="Me",
+            pages=1,
+            size="100kB",
+            submission_date=str(datetime.now()),
+        ))
+
+    @slint.callback(global_name="PrinterQueue")
+    def cancel_job(self, index):
+        del self.printer_queue[index]
+
+    def update_jobs(self):
+        if len(self.printer_queue) <= 0:
+            return
+        top_item = copy.copy(self.printer_queue[0])
+        top_item.progress += 1
+        if top_item.progress >= 100:
+            del self.printer_queue[0]
+            if len(self.printer_queue) == 0:
+                return
+            top_item = copy.copy(self.printer_queue[0])
+        self.printer_queue[0] = top_item
+
+
+main_window = MainWindow()
+main_window.run()