Bring back the WelcomePage, this time as a Document

After some discussion in the SDK sprint, we decided we want it to end up in
as a pinned tab. (and then decide if we like it)
So far, I turned it into a plugin so it can be properly loaded, probably
should move it to kdevplatform.
The Projects Dashboard should be included there.

A patch in Qt is still needed for it to work properly:
https://codereview.qt-project.org/#/c/92201/
Remove the welcome page, moved to kdevplatform
Remove fixme note.
Revert "Try to make the compilerprovider work in Frameworks"

This reverts commit e51b5057baa18ff2eb6f4a3b482efbfb2b2fdc70.

Conflicts:
	languages/plugins/custom-definesandincludes/compilerprovider/compilerprovider.cpp
A better fix, without removing asserts.

Still it's not perfect, but seems like it's Qt bug after all...