Properly extend qt standard widgets, remove the search button in PrefShortcuts.

Properly extend qt standard widgets, remove the search button in PrefShortcuts.
	*  src/frontends/qt4/ui/ShortcutUi.ui: use ShortcutLineEdit
	*  src/frontends/qt4/ui/PrefShortcutsUi.ui: remove searchPB
	*  src/frontends/qt4/CustomizedWidgets.h|cpp: define ShortcutLineEdit and SearchLineEdit.
	*  src/frontends/qt4/GuiPrefs.h|cpp: remove ShortcutEdit
	*  src/frontends/qt4/Makefile.am: build system changes
	*  development/scons/scons_manifest.py

git-svn-id: svn://svn.lyx.org/lyx/lyx-devel/trunk@21193 a592a061-630c-0410-9148-cb99ea01b6c8