A number of changes to make the PGC experience smooth (+ a couple bugfixes)

Requesting @vboettcher to review changes that touch his code (SkyLabeler and CatalogsComponent, especially)

The key changes here are:

1. Refactor how CatalogsComponent::draw works by handling objects of known and unknown magnitudes separately. This allows us to give precedence to labels attached to objects of known magnitude. It isn't yet perfect (i.e. labels are prioritized first by trixel and then by magnitude rather than by magnitude alone), but this is the best we can do without some stronger guarantees on memory availability.
2. Tweak heuristics for displaying objects of unknown magnitude.
3. Tweak labeler heuristics [Reviewers: Check that labels display as expected without PGC catalog]
4. Make find dialog DB queries happen in a separate thread (and not the GUI thread), to keep the UI responsive [Reviewers: Check that Find Dialog works as expected and does not crash]
5. Some minor improvements to the observation planner
6. Change the default policy on SkyMapComposite::findByName to do exact matches. [Reviewers: Check that Ekos works as expected]