[blocked-elements, content-context-menu] Placeholders for other kinds of
elements (onbjects and subdocuments)
[popup] initially hide "no web requests" message
[manager] Sanity checks for manager snapshots
[preferences-rulesets-manager] Save and Reset buttons
Removed old unused code
Replace objects with null-prototype objects (Object.create null) where used as dictionaries
[manager] Cache rulesets by file modification time
"Allow same second-level domain" ruleset
[preferences] Bold current preferences iframe link
[ui/popup] Color Allow/Reject buttons positive/negative
[ruleset] Moved code around, renamed some stuff
[ruleset, preferences-ruleset-manager] Icon showing "permissiveness" of ruleset