Implement single click on line number to select line of text

Summary:
Implements selecting entire line of text with a single click on line number.

FEATURE: 372503

Test Plan:
Click on a line number and check that it selects the line the same way that triple-clicking did before.
Check that selecting a line and then Shift+Click on another line selects all intervening lines.

Reviewers: #frameworks, #kate, #ktexteditor, ngraham, cullmann

Reviewed By: #kate, #ktexteditor, cullmann

Subscribers: dhaumann, rkron, mwolff, ngraham, #ktexteditor, #kate, #frameworks

Tags: #kate, #frameworks

Differential Revision: https://phabricator.kde.org/D11685