Replaced command entry text field with a Scintilla buffer.
`ui.command_entry` now has the same functions and fields as `buffer`s as well as
an additional `height` property. Note that when the command entry is active,
`_G.buffer` is unchanged, so many existing key commands cannot be bound to
command entry key modes. See `keys.lua_command` in
*modules/textadept/command_entry.lua* for the proper way to bind them.
Removed `ui.command_entry.entry_text` and `ui.command_entry.show_completions()`.
Added editing keys for use with other modes; modules/textadept/command_entry.lua
Fixed autocomplete bug in curses; modules/textadept/command_entry.lua
Automatically add standard editing keys; modules/textadept/command_entry.lua