Fix rendering of text input elements having padding by removing the use

of cached parent's proxy style, that end up to return a wrong content rect
size, and using ad-hoc style() function instead. Actually only KLineEdit
inherits KdeUiProxyStyle (since kde 4.5) so the behavior of our internal
proxy is unchanged for the other form elements (i.e m_proxy was always 0 for them).

Also the crash reported in #197270 is no longer reproducible after this change,
likely it has been fixed elsewhere...

BUGS: 280445, 280226
FIXED-IN: 4.8.3