Squashed commit of the following:

commit d1a6823458
Author: AnClark <clarklaw4701@qq.com>
Date:   Thu Dec 15 09:03:18 2022 +0800

    Refactor CMake var _dgl_library -> _dgl_has_ui in plugin build functions

    Plugin build functions (dpf__build_<PLUGIN_TYPE>) invokes
    dpf__add_ui_main() which has a parameter HAS_UI. This parameter acts
    like a switch, controlling if DistrhoPluginMain.cpp shall be compiled.

    Before this patch, value of _dgl_library is passed into HAS_UI. However,
    this will make it ambiguous. Variable _dgl_library should only be served
    as a flag of DGL UI type, and should not be a switch of whether to build
    DistrhoPluginMain.cpp or not.

    What's more, since DPF's CMake build system starts to support external
    UI, which is not limited to DGL, simply checking _dgl_library for
    dgl__add_ui_main() is no longer relevant.

    So, instead, I use variable _dgl_has_ui which keeps to the point. It
    will be set to ON if _dgl_library is non-empty or _dgl_external is ON.

commit 2d162a16b6
Author: AnClark Liu <anclarkliu@outlook.com>
Date:   Tue Dec 13 23:13:49 2022 +0800

    Build ExternalUI and EmbedExternalUI example plugins with cmake

    Note: Build CLAP versions as well.

commit 2cf060910e
Author: AnClark Liu <anclarkliu@outlook.com>
Date:   Tue Dec 13 23:07:40 2022 +0800

    Add external UI support for cmake

Signed-off-by: falkTX <falktx@falktx.com>