Properly save state of KTextEditor Views

Summary:
Call the KTextEditor::View methods for reading & storing the state:

• KTextEditor::View::readSessionConfig()
• KTextEditor::View::writeSessionConfig()

Remove our implementation of storing the state of ...:

• Cursor
• Selection

By using the KTextEditor methods we do save the states of ...:

• Cursor
• Dynamic Word Wrap
• Text Folding nodes
• ...

Thus, with this patch the storing & retrieving of the selection state is
lost. To be honest, I don't see that as a super useful feature anyway.

FIXED-IN: 5.3.0
BUG: 385919

Reviewers: mwolff

Reviewed By: mwolff

Subscribers: mwolff, kdevelop-devel

Differential Revision: https://phabricator.kde.org/D11601