Decouples changes of the current element and changes of the selection.
Introduces `browser.current_card` which has previously been amalgamated
with the previewer card `browser.card`.
The latter triggers `selectionChanged()` unreliably, probably due to the
aggregation of chronologically close events, causing problems in
tracking `_len_selection`. `reset()` never emits signals.
`len(self._view.selectionModel().selectedRows())` is slow for large
selections, because Qt queries flags() for every selected cell, so we
calculate the number of selected rows ourselves.
- Cache the result of 'table.len_selection()'
- Update this cache manually when a row was deleted or restored
- Emit 'dataChanged()' after such a change to fix flags not updating
correctly to the shortcut in 'model.flags()'
- Remove/retsore focus if the current element was deleted/restored
The previous behaviour when a non-sortable column was clicked was to
trigger the sorting routine, which was usually a no-op, but would
reorder rows if the configured sort column was hidden.
