Always preserve line breaks on "Select All" action

Summary:
_preserveLineBreaks was set/reset only in mousePressEvent, and was
initially disabled. This led to use of the last setting from mouse
selection, or initial value, by "Select All".

BUG: 352616

Test Plan:

• Start Konsole
• Run seq 100
• (optional, test should give the same effect with and without this step) Press Ctrl and select some text
• Edit → Select All
• Copy
• Paste in any text editor

Expected result: text should be in multiple lines (like on

Konsole screen)

Actual result: text is pasted as one line

Regression tests

• Run seq 100
• Select few lines using mouse
• Copy
• Paste in any text editor

Expected result: text should be in multiple lines (like on

Konsole screen)

• Run seq 100
• Press Ctrl and select few lines using mouse
• Copy
• Paste in any text editor

Expected result: text should be pasted as one line

Reviewers: Konsole, hindenburg

Reviewed By: Konsole, hindenburg

Subscribers: hindenburg, Konsole

Tags: Konsole

Differential Revision: https://phabricator.kde.org/D12058

(cherry picked from commit 10a41f41b090a485b041c83b891c78aacf034f80)