Access standard application data and asset directories for all supported platforms

[alistair-baxter-kdab]

Plus a series of other additions to the likes of KDUtils::Dir requires to support these, and test them.
Added proper file and directory support for Android to allow reading, and in the case of non-assets writing.
Added a filesystem example, including an Android project, to road-test filesystem functions within a running KDGui application.

The various commits in this PR are intended to be merged by rebase-and-FF rather than squash.

[seanharmer]

Do we need to use signals for the touch events? Can't we subclass in KDGui like we do for xcb or wayland? I'm not familiar enough with Android to know if there are some additional restrictions.

[alistair-baxter-kdab]

Do we need to use signals for the touch events? Can't we subclass in KDGui like we do for xcb or wayland? I'm not familiar enough with Android to know if there are some additional restrictions.

It turns out that the need to do that refactoring was a misconception on my part, so I have just dropped it.

[alistair-baxter-kdab]

Whoops, apparently I don't understand what the GitHub UI buttons do.

[seanharmer]

Looks better to me now. Happy once the linter is silenced.

[MiKom]

I took a look at the assets/ folder situation. I think we can simplify it and contain the folder to the example that it's related to.

How about:

1. move the assets/ folder to examples/filesystem
2. Remove all CMakeListst.txt files in the examples/filesystem/assets
3. Add the following to examples/filesystem/CMakeLists.txt (under add_executable):

add_custom_command(
  TARGET ${PROJECT_NAME}
  POST_BUILD
  COMMAND ${CMAKE_COMMAND} -E copy_directory ${CMAKE_CURRENT_SOURCE_DIR}/assets $<TARGET_FILE_DIR:${PROJECT_NAME}>/../assets
)

With this approach we also won't have all the CMake cruft that's getting generated in the build folder in assets/ right now:

$ ls assets/
CMakeFiles  CMakeLists.txt  CTestTestfile.cmake  cmake_install.cmake  text
mikom@dell-laptop:~/kdab/rnd/kdutils/code/KDUtils/build/Debug$ ls -R assets/
assets/:
CMakeFiles  CMakeLists.txt  CTestTestfile.cmake  cmake_install.cmake  text

assets/CMakeFiles:

assets/text:
CMakeFiles  CMakeLists.txt  CTestTestfile.cmake  asset_text.txt  cmake_install.cmake

assets/text/CMakeFiles:

This also works with multi-config CMake generators like VS or Ninja Multi-Config, unlike the current solution.