Add SetFilterText and SetFilterState

[taigrr]

Currently, there's no way to allow a developer to prefill a list filter reliably. A workaround of using Program.Send works, but is limiting.

This PR introduces two new functions, SetFilterText and SetFilterState which fix this issue and allow for finer control over the list's initial filter state.

Willing to make whatever style changes etc. are required.

[taigrr]

This also addresses #85 but in a different way than #201

[taigrr]

thanks for the tests (both manual and programmatic) @k-x7 !

[taigrr]

@caarlos0 / @maaslalani any chance I could get this revisited?

[lorenzo-milicia]

This implementation does not set the filteredItems list to be the whole list when the state is set to Filtering and the FilterInput has empty string as value. I would add the same code found in the switch to the Filtering state by key press:

if m.FilterInput.Value() == "" {
  // Populate filter with all items only if the filter is empty.
  m.filteredItems = m.itemsAsFilterItems()
}