Store the full relative path to found `.rpi` plugins, relative to the
standard Plugins directory, as specified by wxWidgets.

This fixes the problem of plugins being in a subdirectory while only the
basename was stored, making the plugins unusable.

This is done by using `wxFileName::GetFullPath()` instead of
`wxFileName::GetFullName()` with a relative filename instance.

Make a `GetPluginsDir()` method on the app class to simplify getting
this directory, and for possible future overrides.

Also make some minor, functionally equivalent changes to
`get_config_path()` in `wxvbam.cpp`:

- use the new `GetPluginsDir()` method for the plugins directory when
  building the config file search path

- print the XdgConfigDir on all platforms, since the function works on
  all platforms

- make a `add_nonstandard_path` macro which duplicates the `add_path`
  macro for wxWidgets standard paths but for any arbitrary string path

- use `wxFileName` methods to make the XDG config directory path instead
  of string concatenation

Signed-off-by: Rafael Kitover <rkitover@gmail.com>