Fix paste/cpaste bug and refactor/cleanup that code a lot. In fixing a pasting bug (mishandling of whitespace when input had prompts) it became clear the pasting code hadn't been updated when the new prefiltering machinery was added. Furthermore, the pasting magics are only for the terminal, but the code was in the base classes. This refactors and simplifies the pasting code, moving it to the terminal shell only, and removing unnecessary methods from the main class (using small utility functions instead). The tests were simplified because the previous regexp supported some odd edge cases that are not valid in the normal prefiltering code. We want to have a single location and set of rules for input prefiltering, so I changed some of the test cases to be consistent with what prefilter allows.