Moved textUnderCursor()

Moved textUnderCursor() which returns the current completion prefix from
SnippetCompleter to MarkdownEditor class.
Remove dependency on QPlainTextEdit

Remove SnippetCompleter dependency on QPlainTextEdit. Instead the selected
snippet is signalled to the MarkdownEditor which then replaces the trigger
with the snippet content.
Remove popupOffset from SnippetCompleter

The left offset is instead calculated by MarkdownEditor based
on the width of the line number area.