CreateAppMetaMimeThread: Use open_type()

... instead of constructing the MIME type file path manually.
Remove packages Zlib[-devel] from ReleaseBuildProfiles

Instead add Zlib as a mandatory package and Zlib-devel as a Development
dependency.
Replace various boolean rule parameters by flags

Instead of the boolean alwaysUpdate, isCDPackage, and stripDebugSymbols
parameters use a uniform flags parameter which can contain any of the
respectively supported flags (alwaysUpdate, cdPackage,
stripDebugSymbols, !stripDebugSymbols).
rule CopyDirectoryToContainer: Add flags isTarget

The flags indicates that the passed sourceDirectory is a target, not a
path.
Move MIME DB creation from build_haiku_image to jam build system

* build_haiku_image: Remove MIME DB creation code.
* Rename beos_mime source directory to mime_db.
* Add rules to build the MIME DB in the source directory's jamfile.
* Add MIME DB directory to haiku.hpkg in data/mime_db.
Add private class BMergedDirectory to storage kit

An arbitrary number of directories can be added, which the implemented
BEntryList interface presents as a single merged entry list. Three
different merge policies are supported which define how entries that
appear in more than one directory are treated.
Support multiple MIME DB directories

Each installation location (system, common, common/non-packaged,
~/config, ~/config/non-package) can now have a read-only data/mime_db
directory. ~/config/settings/beos_mime is now named mime_db as well. The
contents of all directories makes up the MIME DB. Entries in more
specific locations shadow entries in more general locations. Only the
directory in ~/config/settings is where the registrar writes changes to.

The new layout allows packages to contribute entries to the MIME DB by
simply providing the respective files in data/mime_db. Consequently the
user settings directory is supposed to contain only the things the user
has actually changed.

Seems to work fine as far as tested. A few issues, though:
* The registrar doesn't monitor the directories yet, so it doesn't
  notice entry changes due to package de-/activation.
* ATM it is not possible to remove a MIME type that is not in the user
  settings directory, although the FileTypes GUI suggests that it is.
  We'd have to work with white-outs, since we cannot remove the files in
  the data/mime_db directories. Or, alternatively, the API has to be
  extended and the FileTypes GUI adjusted to disable the "Remove" button
  in such a case.