cmd_floating: Don't add non-float as sibling to float.
cmd_floating: Don't add non-float as sibling to float.

When turning a float to a non-float, `get_focused_container` might
return another floating view, causing the active view to be inserted
into the floating list on its workspace instead of the normal child list
which it should. (Since it has `is_floating` as false the resulting
discrepency triggered other bad behaviour eventually leading sway to
crash.)

This patch fixes that by simply checking floating status before making
it a sibling.

ascent12/sway,mikkeloscar/sway,christophgysin/sway,1ace/sway,taiyu-len/sway,colemickens/sway,crondog/sway,colemickens/sway,1ace/sway,4e554c4c/sway,4e554c4c/sway,ptMuta/sway,christophgysin/sway,colemickens/sway,ascent12/sway,johalun/sway,crondog/sway,sleep-walker/sway,taiyu-len/sway,SirCmpwn/sway,1ace/sway,ascent12/sway,sleep-walker/sway,mikkeloscar/sway,taiyu-len/sway