Install script with symlinks

Created by: sandeep-khanna

Having git cloned the theme repository, it would be nice to use the installer script to create symlinks to as much of the downloaded content as possible. Doing so, would ensure that all the installed instances of the themes (light, dark, etc) would always have the latest updates when a git pull is performed on the local working directory.

I git cloned this repository in my .themes (~/.themes) folder and run the updated install script (all the cp -ur commands replaced with ln -sf). Since I am running Ubuntu Mate, I didn't make any changes to the install_gdm() function.

With the default destination folder being ~/.themes, the following command creates all the theme variant folders correctly:

./install.sh -n macOS-Mojave

Also, updated my copy of the install script to use a different macOS style icon theme:

IconTheme=[la-capitaine-icon-theme](https://github.com/keeferrourke/la-capitaine-icon-theme)