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/text_input_view.py

@@ -0,0 +1,73 @@
+import curses
+import curses.ascii
+
+from pqcli.ui.curses.event_handler import EventHandler
+from pqcli.ui.curses.views.base_view import BaseView
+
+
+class TextInputView(BaseView):
+    """Ask for one line of text, such as a file name."""
+
+    def __init__(self, screen: curses.window, title: str, text: str) -> None:
+        super().__init__(screen)
+
+        self.on_cancel = EventHandler[[]]()
+        self.on_confirm = EventHandler[[str]]()
+
+        self._title = title
+        self._text = text
+        self._win: curses.window | None = None
+        self._text_win: curses.window | None = None
+
+    def start(self) -> None:
+        self.screen.erase()
+        self.screen.noutrefresh()
+
+        scr_height, scr_width = self.screen.getmaxyx()
+        h = 6
+        w = min(60, scr_width - 4)
+        y = (scr_height - h - 2) // 2
+        x = (scr_width - w - 2) // 2
+        self._win = curses.newwin(h + 2, w + 2, y, x)
+        self._text_win = curses.newwin(3, w + 2, y + 2, x - 1)
+        self._render()
+
+    def stop(self) -> None:
+        self._win = None
+        self._text_win = None
+
+    def keypress(self, key: int) -> None:
+        if key == curses.ascii.ESC:
+            self.on_cancel()
+            return
+        if key == curses.ascii.NL:
+            text = self._text.strip()
+            if text:
+                self.on_confirm(text)
+            else:
+                self.on_cancel()
+            return
+        if key in {curses.KEY_BACKSPACE, curses.ascii.DEL}:
+            self._text = self._text[:-1]
+        elif key == curses.ascii.ETB:  # ^w
+            self._text = ""
+        elif curses.ascii.isprint(key):
+            self._text += chr(key)
+        self._render()
+
+    def _render(self) -> None:
+        if not self._win or not self._text_win:
+            return
+
+        self._win.erase()
+        self._win.addstr(0, 0, self._title)
+        self._win.addstr(5, 0, "[Enter] Continue")
+        self._win.addstr(6, 0, "[Esc  ] Cancel")
+        self._win.refresh()
+
+        width = self._text_win.getmaxyx()[1] - 2
+        self._text_win.erase()
+        self._text_win.box()
+        # keep the end of long paths in view
+        self._text_win.addnstr(1, 1, self._text[-width + 1 :], width)
+        self._text_win.refresh()