SortFilterProxyModel is an awesome piece of code and we use it in several
projects. This is the reason for this patch introducing shared library support.
By doing so we:

* Save memory by using shared library - at least RAM when running multiple
  applications using SortFilterProxyModel
* Save build time: SortFilterProxyModel is build once for all

To remain compatible with previous behaviour user can select what to do by cmake
options:

BUILD_OBJECT_LIB:
(Default ON): Same as before: Add sources to your project and build obj-files
that can be linked to your binary. It was taken care that projects behaves same
as before (tested) - no need for develepers to make changes in their project.

BUILD_SHARED_LIB: Build and install a shared library so that other projects can
use it. This is the way packagers prefer deploying code.

Signed-off-by: Andreas Müller <[email protected]>

Now that SortFilterProxyModel can be build standalone, Qt-Creator creates
CMake.user.. files once it configures SortFilterProxyModel. User configurations
are nothing to be added to repository (accidentally) so add an entry in
.gitignore.

Signed-off-by: Andreas Müller <[email protected]>

This does not cause any change to build but it adds the headers to the file list
of IDEs like qt-creator gives developers much easier access.

Signed-off-by: Andreas Müller <[email protected]>

… --as-needed

There are build environments that link their binaries with --as-needed for good
reasons [1].

This causes trouble here too: The application linking against
SortFilterProxyModel has no reference to a symbol within SortFilterProxyModel
so the library will simply be skipped and not loaded at runtime causing:

| qrc:/qml/FaFilter.qml:2:1: module "SortFilterProxyModel" is not installed

Linker with --as-needed behave similar as linking against static libraries and
Qt documatation [2] warns explicitly not to use Q_COREAPP_STARTUP_FUNCTION when
linking against static libraries.

To get around a stateless class SortFilterProxyModel (luckily this name was not
used yet) was created, that just contains the static method registerQml(). To
avoid multile calls on environemts that do not link with --as-needed all
register functions ensure to be called once only by a variable wasRegistered.

In the following patch Readme.md will updated and will recommend highly to call
SortFilterProxyModel::registerQml() - who knows which environment your
application will be build in?

[1] https://wiki.gentoo.org/wiki/Project:Quality_Assurance/As-needed
[2] https://doc.qt.io/qt-5/qcoreapplication.html#Q_COREAPP_STARTUP_FUNCTION

Signed-off-by: Andreas Müller <[email protected]>

Signed-off-by: Andreas Müller <[email protected]>