Refactorize project files

Every subproject "mysubproject" of type app or lib now contains three qmake
files:

  mysubproject.pro
  mysubproject.pri
  mysubproject_dependencies.pri

Dependency between two projects is satisfied by calling
include(../path/to/dependee/dependee.pri) inside dependent_dependencies.pri.
mysubproject.pri always includes mysubproject_dependencies.pri - this way
dependencies are gathered transitively.

Every mysubproject.pro starts with defining TARGET and including one of the
templates:

  ssuapplication.pri
  ssulibrary.pri

Settings common to both app and lib type of projects comes from

  ssu_common.pri

For tests, there are derived templates inside tests directory

  tests/testapplication.pri
  tests/testlibrary.pri

Common settings similarly comes from

  tests/tests_common.pri

Source code root is added to INCLUDEPATH and DEPENDPATH so it is possible to
include any header file specifying ints path relatively to source code root.
Since all the libs and apps are part of single source tree, I recommend to
express this fact by #including ssu own headers with "" instead of <>.

  #include "libssu/ssu.h"

Projects outside of ssu source tree will still use #include <ssu.h>

For tests, the "tests" dir is also added to INCLUDEPATH and DEPENDPATH so the
"tests/" path component can/should be omitted inside tests code:

  #include "testutils/process.h"

Building:

I have removed the hardcoded build paths (buildpath.pri). It is still possible
to keep source tree clean of build artefacts -- with shadow build:

mkdir build && cd build && qmake ../ssu.pro -r && make

Note that the output directories for app and lib binaries is still hardcoded
as build/{bin,lib}.

When gcov is to be used, simply build without the shadow build and object
files will be placed next to source files.

Other notes:

I found some CONFIG settings having no effect and removed these. Namely

CONFIG += dll - shared libs are build by default
CONFIG -= app_bundle - Mac OS X specific
CONFIG += console - Windows specific
CONFIG += qtestlib - QT+=testlib should be the prefered way

-maybe I am wrong?

The 'unix' conditional was not used consistently. Similar issue with
$${PREFIX}. As it is not much useful at the moment, I rather removed it.
Move SandboxFileEngineHandler from tests to libssu