Add directory drop targets in the file list; fix Copy/Cut/Paste to use the real OS clipboard

Drag-and-drop: directories in the middle file list (list and grid view)
are now drop targets too, same MoveIntoDirectory sidebar bookmarks and
the filetree already use.

Clipboard: Copy/Cut/Paste now goes through Clipboard.set_files/get_files
(the real OS clipboard) instead of private in-process Python state, so
it works between separate SaraFem windows and interoperates with other
apps. Cut-vs-copy has no OS clipboard equivalent, so that one bit lives
in a small ~/.cache/SaraFem/ClipboardMode.txt sidecar every window reads
fresh on pass.

Core.py

@@ -48,6 +48,28 @@ ActiveFontFamily = BundledFontFamily  # set by ApplyAppearance, read by BuildThe
 CacheDir = Path.home() / ".cache" / "SaraFem"
 ThumbnailCacheDir = CacheDir / "Thumbnails"
 
+# Copy/Cut/Paste uses the real OS clipboard (Clipboard.set_files/get_files —
+# see Browser._DoSetClipboard/_DoPaste) instead of private Python state, so
+# it actually works between two separate SaraFem windows (each its own
+# process) and interoperates with other apps' file managers. The one thing
+# the OS file clipboard has no concept of is a cut-vs-copy flag, so that one
+# bit lives here instead, in a tiny sidecar file every SaraFem window reads
+# fresh on Paste rather than trusting its own possibly-stale local state.
+ClipboardModePath = CacheDir / "ClipboardMode.txt"
+
+
+def SetClipboardMode(Mode: str) -> None:
+    CacheDir.mkdir(parents=True, exist_ok=True)
+    ClipboardModePath.write_text(Mode)
+
+
+def GetClipboardMode() -> str:
+    try:
+        Mode = ClipboardModePath.read_text().strip()
+    except OSError:
+        return "copy"
+    return Mode if Mode in ("copy", "cut") else "copy"
+
 # Config moved from ~/.config/SaraFem/ into a SaraSuite-wide layout so other
 # Sara* programs (SaraTag, SaraView) can eventually read from the same place.
 SharedConfigDir = Path.home() / ".config" / "SaraSuite"