Merge #13634: ui: Compile boost::signals2 only once

fa5ce27385 ui: Compile boost:signals2 only once (MarcoFalke)

Pull request description:

  ui is one of the modules that poison other modules with `boost/signals2` headers. This moves the include to the cpp file and uses a forward declaration in the header.

  Locally this speeds up the incremental build (building everything that uses the ui module) with gcc by ~5% for me. Gcc uses ~5% less memory.

  Would be nice if someone could verify the numbers roughly.

  I presume the improvements will be more pronounced if the other models would stop exposing the boost header as well.

Tree-SHA512: 078360eba330ddbca4268bd8552927eae242a239e18dfded25ec20be72650a68cd83af7ac160690249b943d33ae35d15df1313f1f60a0c28b9526853aa7d1e40

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