Refactor UI blocks and use multithreaded scheduler

[frankosterfeld]

Convert the UI blocks RemoteDataSource, SineSource and Arithmetic into "plain" GR blocks that could be moved out of opendigitizer later. This temporarily removes the the remote flowgraph handling for RemoteSource, which will be re-added later.

Also use a multithreaded scheduler, and stop and delete the old graph/scheduler once a new one is set (this required some fixes in RemoteSource and SineSource to avoid freezes/crashes on destruction).

Bump opencmw-cpp to include some fixes for RestClientEmscripten (GET/SUBSCRIBE).

[RalphSteinhagen]

generally OK for this PR but we should move all of these isolated block implementation to the GR 4.0 repo and also make them conform and check the corresponding items in summary issue EPIC: List of GR 3.10 blocks & Others to be ported to GR 4.0

[RalphSteinhagen]

Should be replaced and synchronised with EPIC: List of GR 3.10 blocks & Others to be ported to GR 4.0. Also there is already a sine and cosine source in GR 4.0.

@drslebedev maybe you could comment/document the usage and check this if this conforms to the functionality in GR 3.X.

[drslebedev]

Here is the example of SignalGenerator which can generate sine wave (this is the default signal type), note that ClockSource is required sa input to SignalGenerator:

constexpr std::uint32_t n_samples   = 200;
constexpr float         sample_rate = 1000.f;
Graph                   testGraph;
auto                   &clockSrc  = testGraph.emplaceBlock<gr::basic::ClockSource<float>>({ { "sample_rate", sample_rate }, { "n_samples_max", n_samples }, { "name", "ClockSource" } });
auto                   &signalGen = testGraph.emplaceBlock<SignalGenerator<float>>({ { "sample_rate", sample_rate }, { "name", "SignalGenerator" } });

testGraph.connect<"out">(clockSrc).to<"in">(signalGen);
testGraph.connect<"out">(signalGen).to<"in">(sink);

scheduler::Simple sched{ std::move(testGraph) };
sched.runAndWait();

[RalphSteinhagen]

Looks OK for me from a code-only perspective. Many of the non-UI blocks (and those not dependent on OpenCMW) should become part of the GR 4.0 block-lib. This reduces the duplicate work/documentation/testing. See the EPIC: List of GR 3.10 blocks & Others to be ported to GR 4.0 for details.

Can IMO be merged if @drslebedev and/or @wirew0rm had another chance to functionally cross-check.

[drslebedev]

I added an example of Sine wave generator in the comment to this PR.
The rest looks fine to me and PR can be merged, just keeping in mind in the next iteration to use blocks from GR4.0 where possible.