Nuvolasdk

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* Versioning scheme is synchronized with Nuvola Apps Runtime.
* DBus Launcher has been ported to use the high level Start-up API of Nuvola and a data service have been removed
because it is no longer used.
* Compatibility with Nuvola Player 3.0.x must be enabled with the --compat flag passed to ./configure.
* Web apps scripts are installed into the PREFIX/share/nuvolaruntime/web_apps directory used by Nuvola 4.4+
but legacy symlinks in the PREFIX/share/nuvolaplayer3/web_apps directory are also provided for Nuvola 3.x.

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* Makefile generator: Also install tar-gzipped files in flatpak build mode.
* Makefile generator: DBus Launcher requires Nuvola libraries as it now executes a standalone
Nuvola App Runner process instead of calling the Nuvola Master process to do so. Required
for Flatpak and Snap packaging.
* DBus launcher: Require X11.
* Check project: Check whether licenses are in SPDX format
* appdata: use SPDX format for license field
* appdata: update screenshots and description.
* DBus Launcher: Port to Nuvola 3.1.3.

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* Makefile generator: The `--with-desktop-launcher` option has been removed and the launcher
is always created. Requires Nuvola Player 3.0.7 or 3.1.x.
* Makefile generator: Set a proper StartupWMClass in desktop files.

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* New command 'create-appdata' to create AppStream metadata.
* Makefile generator: Added new ./configure option '--with-appdata-xml' to generate AppStream metadata.
* DBus launcher created a data provider for the flatpak build of Nuvola.
* It is possible to run nuvolasdk as `python3 -m nuvolasdk ...`. This way is now used in generated Makefiles.
* Makefile ends with an erro if the generated metadada.json file is older then the source metadata.in.json file.
* Convert project: Also adds missing icons.

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* check-project: `make clean` isn't run if there is no Makefile.
* data-dir: New command to print the Nuvola SDK data directory.
* Added `examples/home.html` to the data directory. It is a dumb
example of a streaming service to be used in a tutorial.

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* Initial release.