Introduce KDevelop::htmlToPlainText

(cherry picked from commit 95650295fa71d503fe50b2d621f282cc752905d5)

Conflicts:
	util/tests/CMakeLists.txt
Use an estimate of the text width to decide on assistant layout

It used to be based on the amount of items, which caused the significantly
larger, vertical version of the widget to show in cases where it was not
actually required.
Reviewed-By: Kevin Funk

(cherry picked from commit c8f2567579df60b381fb1a91c50b12ad85509aaa)
Make the assistant popup less obtrusive

Don't use the vertical flow layout for *all* items. Only use it for
separating the title and buttons blocks. Always use Flow.LeftToRight for
the *contents* of those blocks
=> Less vertical space wasted for the "create member decl assistant",
   for instance.