SL-20380 Add group notices button as hover button on Groups floater

indra/llui/llaccordionctrl.cpp

indra/llui/llaccordionctrl.h

indra/newview/llgroupactions.cpp

indra/newview/llgroupactions.h

indra/newview/llgrouplist.cpp

indra/newview/llgrouplist.h

indra/newview/llpanelgroup.cpp

indra/newview/skins/default/textures/icons/Group_Notices.png

indra/newview/skins/default/textures/textures.xml

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

...view/skins/default/xui/en/panel_group_list_item_short.xml