Add support for SignalK.

[sebastien-rosset]

This is WIP. It cannot be merged until Android supports API-19.

[rgleason]

@bdbcat @sebastien-rosset Is there a way in CMakeLists.txt to have "if Android set api 118, else set api 119" ?
Then have an if statement in the src code for android, which does not show or allow use of signalK?

[sebastien-rosset]

Yes it should be possible. Though if api-19 support is added soon in the Android fit repo, it might be worth waiting a bit.

[sebastien-rosset]

Currently, OpenCPN requires an exact match between a plugin's declared API version and the supported API versions in OpenCPN's plugin loader. This creates unnecessary restrictions where plugins must target a specific API version even when they could work with newer versions.

    virtual int GetMinAPIVersionMajor() { return GetAPIVersionMajor(); }
    virtual int GetMinAPIVersionMinor() { return GetAPIVersionMinor(); }

// New methods to query host OpenCPN API version
extern "C" DECL_EXP int GetHostAPIVersionMajor(void);
extern "C" DECL_EXP int GetHostAPIVersionMinor(void);

For example, the VDR plugin could implement GetMinAPIVersionMinor() to return 15, meaning older version of OpenCPN are supported. And then the VDR plugin would:

1. Enable NMEA 2000 if GetHostAPIVersionMajor() returns 18 or above.
2. Enable SignalK if GetHostAPIVersionMajor() returns 19 or above.

Of course, this wouldn't work now because none of these functions exist.