Improve the implementation of unix-like directories usage

Add a Meson option "portable" to choose between "portable" and unix-like
directories. Add information about this option in the README.

To determine the user's directory use the variable USERPROFILE only on
Windows and use HOME otherwise.

Implement the "portable" option in the package build script.

README.md

@@ -64,16 +64,32 @@ ninja -C build
 ninja -C build install
 ```
 
+When performing the "meson setup" command you may enable the "portable" option.
+
+If this latter is enabled Lite XL is built to use a "data" and a "user" directory
+from the same directory of the executable.
+If "portable" is not enabled (this is the default) Lite XL will use unix-like
+directory locations.
+In this case the "data" directory will be `$prefix/share/lite-xl` and the "user"
+directory will be `$HOME/.config/lite-xl`.
+The `$prefix` is determined as the directory such as `$prefix/bin` corresponds to
+the location of the executable.
+The `$HOME` is determined from the corresponding environment variable.
+As a special case on Windows the variable `$USERPROFILE` will be used instead.
+
 If you want to install Lite XL on Windows or Mac OS X we suggest to use the script `build-packages.sh`:
 
 ```sh
 bash build-packages.sh <version> <arch>
+
+# In alternative the -portable option can be used like below:
+# bash build-packages.sh -portable <version> <arch>
 ```
 
 It will run meson and create a Zip file that can be easily installed or uninstalled.
 
 Please note the, while compiling Lite XL on Mac OS X should work Mac OS X
-is not well supported.
+is not currently supported.
 
 ## Contributing
 Any additional functionality that can be added through a plugin should be done