Add xdg-desktop-portal support for file dialogs

This patch adds xdg-desktop-portal support for Open/Save file dialogs, allowing Mousepad to use native desktop file dialogs (KDE, GNOME, etc.) instead of GTK's built-in ones.

Changes:

• Convert GtkFileChooserDialog to GtkFileChooserNative
• Set GTK_USE_PORTAL=1 at startup
• Remove encoding selector from file dialogs (encoding can still be changed via Document → Encoding menu)

To apply and test:

• git apply xdg-desktop-portal.patch
• meson setup build --wipe
• ninja -C build
• ./build/mousepad/mousepad

To install system-wide: sudo ninja -C build install

xdg-desktop-portal.patch