Update method & behavior: lineStart/End, not paragraphStart/End

[JordanMartinez]

The methods lineStart and lineEnd move the cursor to the start and end of the paragraph, not the line. Their names should be changed to paragraphStart and paragraphEnd to avoid confusion, as well as renaming selectLine to selectParagraph.

When pressing either Home or End, the caret position should change relative to the current line, not the current paragraph, which is what users have come to expect (e.g., the text area used to write this message has typical home/end key behaviour). There are a few related issues: #384 and #360.

[JordanMartinez]

Apparently, I wrote this in the wrong PR:

This works in all cases except when the caret is on the last line in a multi-line paragraph. When calling lineEnd, it will move the caret to the second-to-last position.

When I accounted for this, it then screwed up caret navigation via the arrows keys in rare cases.

However, it turns out this was an unrelated bug. See #423.