Set the current tab URL correctly when the app is loaded

In Firefox, the app is loaded on-demand only when the panel
icon is activated. Previously the current tab URL was only
set when the user switched tabs or navigated in the current
tab after opening the app.

Copy the current tab URL from the PageAccess interface
on the initial load.
Fix 'new-vault' command failing with relative path

The local filesystem VFS is created with '/' as the root path,
so createVault(fs, <relative path>) tried to create the
vault in '/<relative path>' instead of '<current dir>/<relative path>'.

Resolve the relative path before passing it to createVault()

Fixes #18
Force re-render of app view when window is resized

The main item list view only renders visible items.
When the app container is resized, the list view needs to re-render.
Fix event listener leak in item icon control

Unregister the icon image update listener when an icon
control is unmounted.