Fixing signing error.

- Switched from JPM to WebExtension;
- added respect for secure protocol;
- scoped the whole script;
- scoped the inner stylesheet;
- added ordering of filter by alphabet & type;
- changed filter order from left to right to top to bottom;
- renamed variables ("TempElem" to "TmpElem", "SetNumber" to "DivNumber", "CurDay" to "CrtDay") and CSS class ("ActivityDiv" to "ActivityNames");
- suddenly realized that "Id" should probably be spelt with "d" being lower case;
- removed usage of .btn_small_wide & .btn_small_thin in CSS - while Firefox treats scoped restyling of document-level CSS as scoped, Chrome treats it globally;
- switched usage of for...of to [].forEach.call in ActivityParse() - Chrome doesn't support it for objects yet;
- made several "unframeworking" changes - switched usage of hide() to CSS, $() to document.getElementById(), onclick to addEventListener(), new Element() to document.CreateElement();
- redraw the icons to remove light edges & renamed them;
- updated ReadMe.

Thanks to AMO reviewer Rob W for useful tips.