Improve Filelight installation UX

Before this commit pressing the free space button when Filelight
is not installed would show a singular action called "Install
Filelight to View Disk Usage Statistics…". Pressing this button
would open the store page for Filelight. This is an okay user
experience, but we can do better.

This commit makes it so pressing the free space button when
Filelight is not installed shows an attractive UI that makes clear
that freeing up disk space can be accomplished nicely by installing
Filelight. The "Install Filelight…" button on this UI is connected
to PackageKit directly, so we do not need to show a separate store
like Discover and instead trigger an installation right then and
there. For this, the recently introduced DolphinPackageInstaller
KJob is used.

Installation progress is reported through the status bar similar to
the progress reporting of slowly loading directories or searches.
Installation failure or success is ultimately shown within Dolphin
as a passive notification above the view.

On Microsoft Windows or when PackageKit is not available, the
install button will only open a store page for Filelight.

CCBUG: 477739