Fix wrong preedit cursor position in 2.4.x built on Qt5

[Pavel Sanda]

On Tue, Aug 20, 2024 at 02:02:01PM +0900, Yokota K. wrote:
> LyX-2.4.1 built on Qt5 seems to have wrong cursor position while editing preedit strings using the input method. Since this is annoying for input method users, may I apply the attached patch to the 2.4.x code?
> 
> The preedit cursor position in the completing mode (choosing right chars from candidates by parts) reported by Qt5 points to the *end* of the selected part, while Qt6 points to the *start* of it.
> 
> These different behaviors were not documented, so I missed to reflect it to the Qt5 case in fixing #13054.

Looks reasonable and confined to predit text routines. But why it's only for 2.4.x and not master?

Pavel