fix #1057: Preserve indentation when pasting code blocks - #1059

Fixes #1057.

Pasting indented code could lose its indentation entirely. Reproduced by
driving bpython in a pty and rendering the screen through a terminal
emulator; there turned out to be two independent causes.

Tab events were dropped. just_simple_events() discarded any event
with len(e) > 1 that was not explicitly whitelisted, and <TAB> was
not on the list, so tab-indented code lost all indentation and produced
the reporter's exact error. preprocess.leading_tabs_to_spaces() exists
to convert pasted tabs, but was unreachable because tabs never survived
to reach it.

Runs of spaces were collapsed. compress_paste_event() treats a run
of identical named keys as a held-down key. <SPACE> is a named key in
curtsies, so any read of 8 or more consecutive spaces (the paste
threshold is 7 bytes) collapsed to one. Twelve pasted spaces produced a
single space.

just_simple_events(), is_simple_event() and compress_paste_event()
each kept their own copy of the same list of text-producing keys and had
drifted apart, which is how <TAB> was missed. They now share a
CONTENT_KEYS table.

Fixing those exposed a third problem: a statement at column 0 after a
block was swallowed into it, because pasted code has no blank line to
close the block. separate_blocks() inserts one. It distinguishes a
block awaiting its blank line from a buffer held open by an unclosed
bracket by testing whether appending blank lines makes it compile, and
leaves elif/else/except/finally and comments alone, so
if/else and try/except pastes are untouched.

The script from the issue now pastes and runs correctly. Added 17 tests
covering tab- and space-indented pastes, deep indentation, continuing
clauses, unindented bracket continuations, decorators, multi-line
strings and backslash continuations.