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. Progress is reported through the usual progress reporting of the status bar. Installation failure or success is then shown within Dolphin as a passive notification.
The package installation code here is shared with !775 (merged), so this doesn't just add a whole lot of logic only for this feature specifically.
The changes to the status bar progress reporting API have been separated into !786 (merged) for easier review. I also moved the removal of the DolphinViewContainer::MessageType enum to !790 (merged) for easier review.
@abetts, this is based on your comment !674 (comment 828886). I hope you like it. :)