Previously Waybar only displayed icons for applications which were
installed systemwide. Icons were resolved via `.desktop` files in
directories specified by the environment variable `XDG_DATA_DIRS`.
However the [XDG specification](https://specifications.freedesktop.org/basedir-spec/0.8/#variables)
notes that this variable should only consulted **in addition** to
`XDG_DATA_HOME`:
> `$XDG_DATA_DIRS` defines the preference-ordered set of base
> directories to search for data files in addition to the
> `$XDG_DATA_HOME` base directory.
This because `XDG_DATA_DIRS` contains only systemwide directories,
whereas `XDG_DATA_HOME` contains the userwide directory. Also
including the latter when looking up `.desktop` files allows Waybar to
display icons for applications which where installed userwide.
