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 {key 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 {key 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