Implement IME primitives for PlainEditor. #426

Job	Run time
cargo check (msrv) (windows-latest)	2m 34s
cargo check (msrv) (android) (armv7-linux-androideabi)	23s
cargo check (msrv) (macos-latest)	27s
cargo clippy (android) (armv7-linux-androideabi)	1m 56s
cargo check (msrv) (android) (aarch64-linux-android)	47s
cargo clippy (windows-latest)	4m 11s
cargo check (msrv) (ubuntu-latest)	27s
cargo clippy (android) (aarch64-linux-android)	54s
cargo check (msrv) (android) (x86_64-linux-android)	47s
cargo clippy (macos-latest)	2m 4s
cargo clippy (android) (x86_64-linux-android)	2m 5s
cargo clippy (ubuntu-latest)	1m 23s
cargo doc (windows-latest)	2m 39s
cargo fmt	51s
cargo doc (macos-latest)	1m 5s
cargo test (windows-latest)	3m 28s
cargo doc (ubuntu-latest)	20s
cargo test (macos-latest)	26s
typos	7s
cargo test (ubuntu-latest)	24s
	27m 18s

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