Show input method composition in text fields

[hard25670559]

Description of the problem being solved

Text an input method is still composing is invisible until it is committed, so typing through an IME looks like nothing is happening — no characters, no underline, and the candidate window has no caret to attach itself to. This affects anyone typing Chinese, Japanese or Korean into a build name, note or search field.

This is the application half of the work; the engine half is being submitted to the macOS engine port separately. Related: #5632.

Description of the changes

• Take the composition state the engine reports through a new OnPreedit callback and hold it on main. It is state rather than an input event, since it is replaced wholesale on every keystroke and discarded when composing ends.
• Draw it at the caret, underlined to mark it as uncommitted, in EditControl.
• Report the caret rectangle back so the candidate window can follow the text. A single-line field selects all of its content when focused; that selection collapses to its end once composing starts, since that is where the text will be inserted.

Everything is guarded: SetIMECaretRect is only called if the running engine provides it, and main.imePreedit is nil unless the engine sends a composition. On an engine without IME support this changes nothing.

Steps taken to verify a working solution

Tested on macOS with the system Bopomofo input method and a third-party one:

• Bopomofo symbols and partially converted text now appear underlined at the caret as they are typed.
• The candidate window follows the caret instead of sitting in a screen corner.
• Committing inserts the text at the right place; backspace, arrows and selection behave normally afterwards.
• With an engine that does not send composition events, behaviour is unchanged.