More qt5 port notes

[Boudewijn Rempt]

I tried to do some more porting on the plane yesterday. I fixed a small 
number of todo's, but there are two big things that are really blocking:

* The opengl canvas is broken, mainly because QLibrary now returns a 
QFunctionPointer (which is void (*)()) instead of void*; however, that's 
just the tip of the iceberg. We use that stuff to get the opengl functions 
for e.g. glFenceSync. However, Qt5 has QOpenGLFunctions which actually
abstracts all that way, and we should probably use that instead. 
QOpenGLFunctions is meant to access the functions of desktop opengl in 
a platform-independent way, but it is totally undocumented and I didn't 
have access to google on the plane...

(The Washington-LA plane did have wifi, but I didn't feel it was 
affordable...)

* Action systems. This is weird. Shortcuts work fine in e..g. Karbon, but 
not in Krita. Not even ctrl-o or ctrl-n. But even weirder: we can load 
plugins like imagesplit. They are instantiated, the actions are created, 
the actions are in the menu, the actions are correctly enabled and 
disabled. But selecting the menu entry does precisely _nothing_.

And then I was out of battery...

Boudewijn