Replace tristate checkbox on updates page footer with buttons
Currently the footer uses a tristate checkbox to do many things:
- Let the user select all
- Let the user deselect all
- Show the current status of whether or not all updates are selected
- Show the number of selected updates
- Show the total size of all selected updates
This is way too much work for one UI control! This commit replaces it with two toolbuttons saying "select all" and "deselect all" and a label that shows the total size.