Port to controls2

[Alexander Stippich]

Hello,

I've pushed a branch called controls2_port to origin so everybody can test it. 
It ports everything except the action and buttons and the menu to controls 2, 
i.e. it stays at Qt 5.9.
Most notable visual difference is the metadata view since the new dialog 
component is completely different and it also does not scale correctly right 
now. 
Please test and have a look if I've implemented setting the theme (via 
qtquickcontrols.conf) correctly.
I had to delete some properties that aren't available anymore in controls2, so 
there may be differences in behavior, though I haven't found any yet.
Required frameworks are bumped to 5.41.

Feel free to push any changes directly to this branch, of course.

Best regards,
Alex