Fix double click can only select text within visible region

Summary:
This is a reimplemented 914067d1 (hinted by @cfeck), plus a refactoring of TerminalDisplay::findWordStart()

findWordStart() had bounds check issues and an off-by-one for the case when the acquired region was more than two screens down.

TerminalDisplay::findWordEnd() is very similarly implemented, but I haven't found a case where it fails, so I haven't touched it.

Since 914067d1 caused a revert, I've tested the changes thoroughly (the cases are in the Test Plan below).

BUG: 399109

Test Plan:
Corner cases tested (everything was run under valgrind, since I had no crashes without it pre - f98c752b):

- At the end of the word, current Konsole doesn't select "@" at the end of the word when double-clicking, but selects it when the mouse is moved. With the introduced changes the behavior is to consistently ignore the last "@" character.
- When 10K lines buffer is underfilled, dragged doubleclick+scrolled up by dragging the mouse above the terminal (this generates valgrind warnings on 5a31b4af that might have caused Konsole to crash)
- Same with overfilled 10K lines buffer.
- With decreased window size and some chars hidden, dblclick and drag up and down to the end of the buffer. The ends of the string are trimmed (like in current console), the rest of the string is properly selected.
- With increased window size and padded chars, prod the buffer limits. Long lines are broken into separate lines -- no difference vs current Konsole.

Reviewers: #konsole, hindenburg, cfeck

Reviewed By: #konsole, hindenburg

Subscribers: ngraham, cfeck, konsole-devel

Tags: #konsole

Differential Revision: https://phabricator.kde.org/D16159