A split version of D8168. This only refactors the hardcoded ui constructor
into an equilvalent UI file. This change has no string change nor behavior
I tested it and everything seems fine. But can we also get rid of the special "General" tab, i.e. the kmousedlg.ui file with its related KMouseDlg class, by integrating it into the kcmmouse.ui file like the other tabs? The class seems to be some unfinished remnant of a previous attempt of putting tabs into ui files.