Drop Qt4 support

[SunBlack]

Fixes #2618

I didn't modernized any C++ code in this PR to keep PR small ;)

[taketwo]

Thanks for pushing this on. Two points from my side:

1. The (now removed) pcl_find_qt5.cmake script included the following Qt modules:

• Qt5Core
• Qt5Gui
• Qt5Widgets
• Qt5Concurrent
• Qt5OpenGL
  With your change we only search for OpenGL and Widgets components. Could you please comment on the rest?

2. Style: should we maybe use lowercase names for functions/macros, e.g. qt5_wrap_ui?

[SunBlack]

Qt5Concurrent didn't seems to be used and Qt5Widgets has as dependency Qt5Gui (and Qt5Core) and QtGui has dependency to Qt5Core. Find script of Qt5 respect this, so we don't need to search explicitly for all dependencies. Furthermore: Imported targets can define dependencies, so if we call target_link_libraries with Qt5::Widgets, Qt5::Gui and Qt5::Core are implicit linked, too.

I will remove qt5_wrap_ui and other qt5_* calls later. Instead I will switch to CMAKE_AUTOUIC, ... This is just not part of this MR, because I have then to move ui files to another location (they have to be in same directory like cpp file) and change include paths from e.g. #include <pcl/.../ui_foo.h> to #include "ui_foo.h". See here

[taketwo]

Thanks for clarifications!

This is just not part of this MR, because I have then to move ui files to another location

👍 makes reviewing easier

[SunBlack]

Just saw: We have to adjust Azure pipelines. PCL_QT_VERSION is not necessary anymore and I don't know why, but build servers don't find Qt5 (works on my machine, so no idea why it isn't working on Azure)

//Edit: Fixed in #2719