Improves IME ESC key handling

[SadPencil]

This is a small update. It basically keeps the original implementation in 88185ee, but the condition of clearing the textbox is changed: only if HandleEscapeKey() returns false, i.e., this user has never used IME.

Explanation:

        // This method disables the ESC handling of the TextBox as long as the user has used IME.
        // This is because IME users often use ESC to cancel composition, and even if currently the composition is empty,
        // the user still expects ESC to cancel composition rather than deleting the whole sentence.
        // For example, the user might mistakenly hit ESC key twice to cancel composition -- deleting the whole sentence is definitely a heavy punishment for such a small mistake.

To make this judgement acts correctly, this PR intends to remove IMEEventReceived = true line in OnIMETextInput() method, as this method will also be called even if the user does not use an Asian IME.

Tested on my machine.