Uses the built in cmake -Dtest=ON

It should download googletest on the fly if needed.

Adds a new make target, `make test`

Ref #167

fix #1547

 * Removed remaining picture handling from card database and cardinfo
 * removed the “noCard object” hack

Use -DPORTABLE=1 to build a portable version

This is the first implementation of a theme manager. It’s a rebased and
revisited version of #792.

 * added missing bits of serverside code;
 * added fronted in client;
 * removed demo python scripts;

This is the first implementation of a theme manager. It’s a rebased and
revisited version of #792.

 * add to the application bundle a custom info.plist
 * fix a graphic glitch on the life counter

 * ignore .tx folder
 * cmake: when UPDATE_TRANSLATIONS=ON, extract new strings only to the
english translation file.
 * added info in CONTRIBUTING.md
 * updated english translations

 * Always install “release” plugins
 * install only plugins actually used

We previously parsed only .cpp files, but some strings are inlined in
header files.

fixes #411

They are needed to fetch prices from deckbrew and possibly, in the
future, for ssl server connections

* Move all cmake-related files in a new “cmake” folder
* move nsis files in there, too, since they are templates parsed by cake
* retrieve git version once when cmake is run, and use it for both
creating version_string.h/cpp and for package naming

You won’t have to copy manually all the dlls in the release directory
anymore