Fix hard dependency on Widgets package when building modules that don't need uic

While trying to port qtimageformats to CMake, if I don't specify a find_package(Qt5Widgets), the configuration process fails with a bunch of errors like:

CMake Error in src/plugins/imageformats/wbmp/CMakeLists.txt:
  Error evaluating generator expression:

    $<TARGET_FILE:Qt5::uic>

  No target "Qt5::uic"

This is because we automatically enable "uic" autogen for all qt targets via qt_autogen_tools() function, unless the DISABLE_AUTOGEN_TOOLS option is specified.

We shouldn't have to specify that option in so many places though.