[2.0] Off-workspace window-list alerts

For whatever reason 1232 will not update with rebased commits, so I'm opening a new request.

Copy-pasted from #1232:

- Added option in Cinnamon Settings to disable window alerts
- Added blinking effect to window list item icon, to complement the style change when a window demands attention
- Added off-workspace notification.  When a window demands attention, its window-list item will appear on the current workspace.  Clicking on the item will switch to that window's workspace and focus the window.

Addresses: 
#1063
#1234
#1084
#147

FOR DISCUSSION:

I *really* think that windowsAttentionHandler class should be entirely disabled, and that we should react to demands-attention, and window-marked-urgent signals from the WM entirely from this applet.  Right now we only listen for window-marked-urgent in this applet, and windowsAttentionHandler listens for demands-attention.  Applications, it seem, use these 2 interchangeably.

Yes it may result in an additional action required when the USER clicks a link to open in an off-screen window, but at least the control is in the user's hands.  windowsAttentionHandler takes the decision out of the user's hands in many cases, often when it is undesirable.