[Ericky14]

Problem

A finger does nothing in a text_editor. On a touch laptop or a handheld, a tap in Slate's note body neither focuses it nor moves the caret, and there is no way to select text. text_input already handles touch.

Fix

• A tap focuses the editor and puts the caret where the finger lands.
• Two quick taps select a word, three select a line.
• Dragging a finger selects, and stops at the editor's edge. A tap outside unfocuses it.
• Ported from upstream iced-rs/iced#3442. No API change.

Testing

• New widget/tests/text_editor_touch.rs (5 tests, touch events through the simulator): all pass; all 5 fail on master.
• cargo lint and cargo fmt --check pass.
• Slate against this branch, mount tests skipped: on tiny-skia only file_types fails, on the same 21 pptx, pdf and mp4 captures main gets wrong on this machine (LibreOffice and poppler). On wgpu, goldens and file_types failed under load in the full run and pass alone, as they do on iced master. No golden moved.

To try it:

1. cargo test -p iced_widget --test text_editor_touch
2. On a touch screen, tap, double-tap and drag in Slate's editor. Before this change nothing happens.

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