Installer: CopyEngine: Remove some debug/uncommented code
Installer: Move path string constants to new InstallerDefs.h/cpp
Installer: Remove "/boot/var" directory check

The PM Haiku Installer doesn't need support installing ancient Haiku
versions.
Installer: CopyEngine: Externalize decision making

* Add interface EntryFilter, an instance of which can be passed to the
  CopyEngine. The object is asked whether to copy entries/clobber
  directories.
* Move the _ShouldCopyEntry()/_ShouldClobberFolder() code to new
  WorkerThread::EntryFilter.
Installer: WorkerThread::_PerformInstall(): get rid of goto
Installer: WorkerThread: Remove InstallerWindow dependency

* Move message constants to InstallerDefs.h.
* Determine the source and target partition ID already in
  InstallerWindow and pass those to WorkerThread instead of fiddling
  with menu items in _PerformInstall(). And instead of the window object
  pass a messenger to the constructor.
Installer: CopyEngine: Pass relative path to EntryFilter

... instead of the file name.
Installer: WorkerThread::EntryFilter: Use path map

... instead of implicit comparisons.
Installer: Restrict entry filter to files from the BFS volume

We generally want to skip the contents of the packagefs volumes (save
for the shine-through directories). That makes Installer usable again.
In what direction we want to develop it (e.g. integrate some PM support,
so that a subset of packages can be selected) needs further discussion.