Enable selection and window capture on Wayland via interactive XDG portal

[luigimassa]

Summary

On Wayland, Shutter could only capture the full screen through the XDG desktop portal. The Selection, Window and Active Window modes were disabled because they rely on X11 (Wnck/GdkX11), which cannot enumerate or grab other windows under Wayland's security model (#187).

This PR routes those modes to the portal's interactive mode instead: the compositor (e.g. GNOME) shows its own native picker (area / window / screen) and returns an already-cropped image into Shutter's existing post-capture flow.

Changes

• Wayland.pm: xdg_portal() takes an $interactive flag and adds the interactive boolean option to the Screenshot portal request (typed via Net::DBus::dbus_boolean). The full-screen request is byte-for-byte unchanged. A portal Response of 1 (user cancelled) now maps to error code 5 (aborted by user) instead of 9, so cancelling the picker shows no error dialog.
• bin/shutter:
  • the Wayland guard now lets full/select/window/awindow through;
  • the capture dispatch routes select/window/awindow to xdg_portal($s, 1) on Wayland;
  • the Selection and Window buttons/menu items (toolbar and tray) are enabled on Wayland.

Out of scope (Wayland limitations)

• Menu and tooltip capture stay disabled (not supported by the portal).
• Window-by-name/regex (--window=PATTERN) and fixed-geometry selection (-s=X,Y,W,H): the portal accepts neither a target nor coordinates.
• The workspace/window dropdown menus still require X11 to enumerate.

Testing

• perl -c passes on both modified files.
• Manually verified on a GNOME Wayland session: Selection and Window buttons open GNOME's native picker, the captured image lands in Shutter's normal flow, and cancelling the picker produces no error dialog.
• Xorg and full-screen Wayland behaviour unchanged.

Refs #187