Bump imgui to 1.7.9.

Follow upstream update, update imgui to 1.77.

Rename the tag of a release according to reasonable versioning.

Change log

1. Msgbox now has only two parameters. And provides Msgboxv for more control.
2. Fix issue that Msgbox cannot be open from menu.
3. Expose SetClipboard and GetClipboard from Platform.
4. Add DatePicker.
5. Wrap ColorfulTextEditor with minimal functions.