Fix word selection mode

Test Plan:
* run `printf '%.0skonsole ' $(seq 200)` to generate some words
* double click on the beginning of any word
* move mouse pointer right/bottom to extend selection

Expected result: the selection should extend word by word, with the
selection end being on the left or right side of a word located under
pointer

Actual result: right selection end extends character by character and
is located at fixed offset from the pointer

Reviewers: #konsole, hindenburg

Reviewed By: #konsole, hindenburg

Subscribers: hindenburg, ngraham, konsole-devel, #konsole

Tags: #konsole

Differential Revision: https://phabricator.kde.org/D12939