(4.x) dnn: fix API - explicit ctors, const methods

[alalek]

relates #21429

Validate:

• Halide build configuration
• OpenVINO build configuration
• (FAILED) OpenVINO build configuration (NN builder) - gapi build failed with OV 2019r3.0
• Vulkan
• CUDA

force_builders=Custom,Custom Win,Custom Mac

buildworker:Docs=linux-4,linux-6
build_image:Docs=docs-js:18.04

Xbuild_image:Custom=ubuntu-vulkan:16.04
Xbuild_image:Custom=ubuntu-cuda:18.04
Xbuild_image:Custom=halide:16.04
Xbuild_image:Custom=ubuntu-openvino-2019r3.0:16.04
build_image:Custom=ubuntu-openvino-2021.4.2:20.04
Xbuild_image:Custom=ubuntu-openvino-2021.3.0:20.04
Xbuild_image:Custom=ubuntu-openvino-2021.2.0:20.04
Xbuild_image:Custom=ubuntu-openvino-2020.3.0:16.04
build_image:Custom Win=openvino-2021.4.2
Xbuild_image:Custom Win=openvino-2021.3.0-vc16
build_image:Custom Mac=openvino-2021.4.2

test_modules:Custom=dnn,python2,python3,java
test_modules:Custom Win=dnn,python2,python3,java
test_modules:Custom Mac=dnn,python2,python3,java

buildworker:Custom=linux-1
Xbuildworker:Custom=linux-6
Xbuildworker:Custom=linux-5
test_opencl:Custom=ON
test_bigdata:Custom=1
test_filter:Custom=*

buildworker:Custom Win=windows-3
test_opencl:Custom Win=ON
test_bigdata:Custom Win=1
test_filter:Custom Win=*

[alalek]

👍