Multi-press support

[ickshonpe]

Objective

The text input widget selects the word under the pointer on a double click. This isn't correct, the select word edit should be queued on the press following a click. This allows for users to select a word with a double press and then hold the button and drag to extend the selection.

Solution

Track multi-presses as well as multi-clicks:

• Add a count field to Press events.
• Update the click count in PointerButtonState::clicking on consecutive presses within the multi-click duration, not on releases.
• Filter the clicking map in both the Press and Click dispatchers.

Testing

cargo run --example multiline_text_input

You should observe that dragging after selecting a word with a double press extends the selection.

It's still not quite right as the selection is extended from the point in the word that was clicked, not the word itself. This is a bug in Parley revealed by this change.