Add command for sticky-shift

Some SKK users prefer sticky-shift, in other words, to use some key (in my case, `@`) to start preedit(`▽`).

I found `start-preedit(-kana)` is the only available way to do that in libskk, but it is not satisfiable.
When typing in the middle of line, `start-preedit` has side effect to change the following to preedit.

From the command name, `start-preedit` seems to just start preedit, without re-conversion.
However, changing command names and behaviors has too large effect, then I added new command `start-preedit-no-delete`.
I agree that the name is poor, but wish this function is appreciated.