Rewrite initial applet layout to prevent position desync

[serebit]

Description

Rather than adding applets directly to the panel on initial panel load, the load_applets function instead uses intermediate region lists sorted by the info's desired position. Once all applet info have been added to the region lists, each applet's info is updated with its actual position within the list, and is then added to the panel. This prevents issues with applets being uninstalled before being removed from the panel, and thus should resolve #72, along with other applet ordering issues that could be the result of initial race conditions.

Submitter Checklist

• Squashed commits with git rebase -i (if needed)
• Built budgie-desktop and verified that the patch worked (if needed)

[JoshStrobl]

Only the first review item is a blocker. Kinda just pointless gendering of an applet and doesn't really add any value to the documentation.

The later review item I can't say I care much about. Vala memory management is lacking anyways.

[JoshStrobl]

Thanks for the excellent work on this. Validated locally as well and did not spot any regressions. 💯