Easier lookup

The focus of these changes are to make looking things up easier and faster.

To summarise the changes:

* Open/Close the CanIUse panel with the "Ctrl-Alt-U" keyboard shortcut.
* Filter on the selected text if any and focus the filter field when opening the panel.
* Updated design of the filter and feature list column.
* Fix a bug where data was loaded each time the panel was opened, causing duplicate entries in the feature list.
* Hide category titles when there are no matches in the category.
* Keyboard navigation for filter and feature list; Tab/Shift-tab or Up/Down or J/K.
* Trim the filter value from whitespace at its beginning and end.