ui: import and export .pqw files from the menus

The basic interface gets two main menu entries that ask for a path.
The curses roster gets [I] Import and [E] Export, with a new text input
view for the path, prefilled with NAME.pqw on export, and a message
view for the outcome and any warnings. The README explains how to move
characters to and from the web version.

Resolves #21

pqcli/ui/curses/views/message_view.py

@@ -0,0 +1,53 @@
+import curses
+import curses.ascii
+import textwrap
+
+from pqcli.ui.curses.event_handler import EventHandler
+from pqcli.ui.curses.util import KEYS_CANCEL, Choice
+from pqcli.ui.curses.widgets import Menu
+
+from .base_view import BaseView
+
+
+class MessageView(BaseView):
+    """Show a message until it is dismissed."""
+
+    def __init__(self, screen: curses.window, message: str) -> None:
+        super().__init__(screen)
+
+        self.message = message
+        self.on_close = EventHandler[[]]()
+
+        self.choices: list[Choice] = [
+            Choice(
+                keys=[curses.ascii.NL, *KEYS_CANCEL],
+                desc="[Enter] OK",
+                callback=self.on_close,
+            ),
+        ]
+
+    def start(self) -> None:
+        scr_height, scr_width = self.screen.getmaxyx()
+        self.screen.erase()
+        self.screen.noutrefresh()
+
+        width = max(20, scr_width - 6)
+        header = "\n".join(
+            "\n".join(textwrap.wrap(line, width) or [""])
+            for line in self.message.split("\n")
+        )
+        self.main_menu = Menu(
+            header=header,
+            choices=self.choices,
+            active_choice=0,
+            scr_height=scr_height,
+            scr_width=scr_width,
+        )
+        self.main_menu.render()
+
+    def stop(self) -> None:
+        self.main_menu.stop()
+
+    def keypress(self, key: int) -> None:
+        self.main_menu.keypress(key)
+        self.main_menu.render()