a11y: Actually return line when requested

Previously, just the data for the word were returned for the QAccessible::TextBoundaryType::LineBoundary case, just like for QAccessible::TextBoundaryType::WordBoundary.

The QAccessible::TextBoundaryType::ParagraphBoundary case already does what's needed, so use that for QAccessible::TextBoundaryType::LineBoundary as well.

This makes the Orca screen reader announce the whole line when moving between lines using the up/down arrow key, instead of just annoucing the word. (As a side note, the announcement is not 100% reliable, but that's unrelated to this change.)