Preserve double/triple click mode when extending the selection

Summary:
When extending the selection using the mouse the double or triple click
mode that initiated the selection should be preserved. If the user double
clicked to select a word, then extending the selection should extend
"word at a time" rather than "character at a time". Similarly if the user
triple clicked to select a whole line initially, then extending the selection
should extend "line at a time" rather than "character at a time".

BUG: 374454
FIXED-IN: 18.12

Reviewers: Konsole, hindenburg

Reviewed By: Konsole, hindenburg

Subscribers: konsole-devel

Tags: Konsole

Differential Revision: https://phabricator.kde.org/D16660