[Lazarus] Minor typo in rttigrids.pas and objectinspector.pp

[Howard Page-Clark]

I've noticed that the property PrefferedSplitterX is misspelled in three 
source files (should be PreferredSplitterX). The corresponding "F" data 
field that holds the value is spelled correctly, so it's clearly an 
inadvertent typo.

Locations are:
lazarus\components\rtticontrols\rttigrids.pas (74,14)
lazarus\debugger\frames\debugger_general_options.pas (274,5)
lazarus\ideintf\objectinspector.pp (488,14) (536,14) (3745,62) (3787,10)