Add user-level install script for immutable distributions

[lazydays79]

Problem

The existing installation scripts (install.sh, Easy_Install.sh,
Interactive_Install.sh) all write to /usr/share, which is read-only on
immutable distributions such as Fedora Silverblue, Bazzite, and Vanilla OS.
This makes it impossible to install the wallpapers on those systems without
workarounds like rpm-ostree overlay or similar.

Solution

This PR adds a new script install_user.sh that installs all wallpapers
into the user's home directory (~/.local/share/) instead of /usr/share/.
GNOME picks up wallpaper definitions from both locations, so the wallpapers
appear in Settings → Background exactly as with a system-wide installation.
Since all XML files contain hardcoded /usr/share/backgrounds/Dynamic_Wallpapers
paths, the script rewrites every XML file on the fly using sed before writing
it to the destination — no pre-patched files are needed in the repository.
No sudo or root access is required at any point.

Changes

• install_user.sh (new): Installs wallpapers into
  ~/.local/share/backgrounds/Dynamic_Wallpapers/ and picker XMLs into
  ~/.local/share/gnome-background-properties/. All hardcoded /usr/share
  paths in both slideshow XMLs and picker XMLs are rewritten on the fly.
  Supports --uninstall to cleanly remove only the files placed by this script.
• README.md: Added Method 4 section documenting the new script,
  following the style of the existing installation methods.

Tested on

• Standard mutable Linux (e.g. Fedora Workstation, Ubuntu)
• Immutable Linux (e.g. Fedora Silverblue, Bazzite, Vanilla OS)