Refactor build system

The client was built with Meson, the server with Gradle, and were run by
a Makefile.

Add a Meson script for the server (which delegates to Gradle), and a
parent script to build and install both the client and the server to the
system, typically with:

    meson --buildtype release build
    cd build
    ninja
    sudo ninja install

In addition, use a separate Makefile to build a "portable" version of
the application (where the client expects the server to be in the
current directory). Typically:

    make release-portable
    cd dist/scrcpy
    ./scrcpy

This is especially useful for Windows builds, which are not "installed".