Initial start of variables dialog

Summary:
This patch adds one public function to KTextEditor::Editor:

void addVariableExpansion(const QVector<QWidget*>& widgets,
                          const QStringList& variables = QStringList()) const;

Once called for QLineEdits (QTextEdit support still pending),
QLineEdits will get an action that opens an variable expansion
dialog that allows the user to insert variables from a list
view.

What is still missing:

• add support for QTextEdits
• use for backup prefix/suffix (bug 403583)
• use in External Tools plugin (bug 299248)

Test Plan: make && make test

Reviewers: cullmann

Reviewed By: cullmann

Subscribers: kwrite-devel, kde-frameworks-devel

Tags: Kate, Frameworks

Differential Revision: https://phabricator.kde.org/D23824