Multiline support for BaseStylesheet property

Adds (using assembly qualified names) the EditorAttribute to the
BaseStylesheet property of the HtmlLabel, HtmlPanel and HtmlToolTip
controls, allowing the use of a multiline editor instead of being
constrained to a single line.
Set to v2 instead of v4.
Merge pull request #10 from cyotek/BaseStylesheet-Multiline

BaseStylesheet Property Mutliline Edit Support