This class had slightly confusing semantics and the added weirdness
doesn't seem worth it just so we can say "." instead of "->" when
iterating over a vector of NNRPs.
This patch replaces NonnullRefPtrVector<T> with Vector<NNRP<T>>.
This commit allows the text tool's internal TextEditor component to
handle keyboard shortcuts that would normally be handled by menu
actions.
The shortcuts that can now be used are: cut, copy, paste, undo, redo
and select all.
Previously, we were rewriting the on_primary_color_change in the Text
Tool and Gradient, which made the palette widget no longer update after
picking a color from an image. Additionally, it also crashed the program
after leaving the Gradient tool and trying to change color.
