fix(CMake) add options, includes and dependencies

[higaski]

• Added options to modify the following definitions
  • LV_DEMO_CONF_INCLUDE_SIMPLE, default: ON
  • LV_DEMO_CONF_PATH, if defined adds parent folder to include path

    # Project which includes lvgl sets LV_DEMO_CONF_PATH
    set(LV_DEMO_CONF_PATH
    ${CMAKE_CURRENT_SOURCE_DIR}/src/lv_conf.h
    CACHE STRING "" FORCE)

• Added dependencies
  • lv_demos depends on lvgl
• Namespaced target aliases
  • lvgl::demos
• Applied cmake-format for a consistent style

This PR is also part of #2753

[kisvegabor]

Thanks!