removed picture_style from LLButton::Params and allow empty labels

indra/llui/llbutton.cpp

indra/llui/llbutton.h

indra/llui/llfloater.cpp

indra/llui/llflyoutbutton.cpp

indra/llui/llmenugl.cpp

indra/llui/llscrollbar.cpp

indra/llui/lltabcontainer.cpp

indra/newview/llchiclet.cpp

indra/newview/lldebugmessagebox.cpp

indra/newview/llfavoritesbar.cpp

...ewview/skins/default/xui/en/floater_animation_preview.xml

indra/newview/skins/default/xui/en/floater_avatar_picker.xml

indra/newview/skins/default/xui/en/floater_camera.xml

indra/newview/skins/default/xui/en/floater_color_picker.xml

indra/newview/skins/default/xui/en/floater_gesture.xml

indra/newview/skins/default/xui/en/floater_lagmeter.xml

indra/newview/skins/default/xui/en/floater_media_browser.xml

indra/newview/skins/default/xui/en/floater_moveview.xml

indra/newview/skins/default/xui/en/floater_report_abuse.xml

indra/newview/skins/default/xui/en/floater_test_button.xml