Fix cursor movement of string input popup

[Qrox]

Summary

SUMMARY: Bugfixes "Fix cursor movement of string input popup"

Purpose of change

Fix cursor movement of string input popup. For example, fix moving cursor left past the start of the displayed text causing the text to be displayed from the very beginning until cursor is moved right.

Describe the solution

1. More precise string length and display width comparisons.
2. Make display shift a class member instead of local to query_string.
3. Erase non-custom string input popup window to clear IME preview text.
4. Clean up some string operations and comments.

Testing

Moved cursor back and forth a couple times, with English or Chinese text, and it worked fine.

[BrettDong]

I suggest that we shift the string by input box width when it's currently displaying some middle portion of the string and user moves the cursor to the left to view the left part of the string.

[actual-nh]

I suggest that we shift the string by input box width when it's currently displaying some middle portion of the string and user moves the cursor to the left to view the left part of the string.

The entire input box width, or almost that (to allow for seeing context, such as that there wasn't any part being skipped over)?

[Qrox]

The entire input box width, or almost that

I went with 1/5 of the text area width or 10 characters, which ever is shorter.