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

 If none of the gameboard images is set and and the user’s data
directory doesn’t contain a “zonebg” directory, create the directory,
copy contents from cockatrice’s app bundle and sets the config
accordingly.