Use solver to check for problems and produce a list of package transactions

Use solver to check for problems and produce a list of package transactions

Convert chooser UI selections into a SolverTaskList

Apply SolverSolution to that task list (with choice of keep, upgrade,
upgrade with test, IncludeSource) to produce a vector of SolverTransactions.
Store a solution object in packagedb

The transaction list returned by the solver is postprocessed to add
reinstall and IncludeSource actions

Very crudely present solver problems in the PrereqChecker page UI, as text.
Change tickbox to say "accept default solutions" and don't allow to preceed
unless those solutions are accepted (ideally we would have a UI to choose
solutions).  Remove warning about missing dependencies.

Also pass initial trust state to PrereqChecker

v2:
Fix comment typo (kbrown)
Tweak comment in OnActivate() (kbrown)

v3:
Drop stray \n at end of report() output (kbrown)