Selectmenu: Introduce _renderButtonItem() method

Fixes #10142
Closes gh-1299
Sortable: Redetermine floating flag when recalculating positions

This addresses a bug where users initialize empty sortable lists are
add items dynamically. In this situation refresh() should recognize the
position and orientation of the new items.

Fixes #7498
Closes gh-1381
Selectmenu: Properly parse value from options

Fixes #10684
Selectmenu: Copy title attributes

Fixes #10435
Closes gh-1298
Tabs: Suppress automatic activation when navigating with COMMAND

Fixes #9621
Closes gh-1383
Droppable: Remove $.ui.intersect()

Fixes #10534
Demos: Fix menu markup in tooltip video-player demo
Demos: Fix menu markup in split button demo