Clean up project structure and code

There are some things that could be done to simplify the project structure and code, making it easier to read and more friendly for presenting for interested parties, due to better readability and less warnings.

• flatten up the project structure - currently lots of redundant subdirectories
• move C++ code outside of the DS generated QML module directory (I think App directory is the recommended/default way now? For that, e.g. the Industrial Automation demo (https://git.qt.io/qt-demo-team/industrial-automation-hid/Development/responsive-scada/-/tree/world-summit-2025 ) can be used as an example, other option also visible there is a plain, separate "src" directory
• move "functional" QML code out from .qml.ui files to .qml files
• Flex: replace the "buttons" implemented with Rectangles and Mouse Areas with actual Buttons so as to not have to re-implement the whole clicked/released etc. functionalities