Window activation states

[jvonitter]

• A window may be active or inactive. Only one window may be active at a time and there must always be one inactive window.
• When another window is activated (by clicking anywhere in the window) all others are inactive.
• When a window is active, window controls for close, move left and move right (UIS-70 100) appear above the window and the URL bar/buttons appear below it.
• When a window is inactive, window controls are not displayed for that window.

Consider whether GeckoView changes are needed.

Multi-window spec

Needed by #1319

[keianhzo]

Closed by #1467