Paragraph+TextDocument: Added GetText() method

 * GetText() extracts the text from a certain range as BString.
 * Probably fixed TextDocument::ParagraphAt(), though it is still untested.
TextDocumentView: Added plain-text copying to the clipboard

 * Make the focus view on MouseDown()
 * Also added reaction to focus and window activation changes.
HaikuDepot: Show message when no package is selected.

the instable layout was irritating. Clicking a package and having the
package info area expand upward could even lead to the clicked packed
to be hidden in the listview. Now a message is displayed and the package
info area already has the correct height.