Improve keyboard usability in Qt GUI

Add a number of standard keyboard shortcuts to menu items in the Qt UI,
especially under the File menu. To make action names more conventional,
rename "Create/Restore" to "New/Restore", "Copy" to "Save Copy", and
"Close" to "Quit". This also resolves the situation where
Create/Restore, Copy and Close all had "C" as their mnemonic.

Also allow fast switching between tabs by using Alt+[1-5], same as
in tabbed web browsers. Ctrl+PgUp/PgDown is still available.