Rewrite and move windowForTab to a utils module to fix GM_openInTab.

This was broken by d2822b79eaec63ab2f8aa7646a82bdf3350a312e.
[WIP] Move script injection into frame script for e10s.
[WIP] Add some TODO comments.
[WIP] Pass URL to sandbox
[WIP] GM_rmc
[WIP] Remove aForce from withAllMenuCommandsForWindowId
[WIP] Remove unused/broken GM_BrowserUI observer
[WIP] Remove unnecessary page{hide,show} listeners in GM_BrowserUI
[WIP] GM_openInTab