Add double-click-and-drag and triple-click-and-drag text selection

[mudbungie]

• Closes Add double-click and drag and triple-click and drag to select multiple words and to select multiple paragraphs #2550
• I have followed the instructions in the PR template

Double-click-and-drag now extends the selection by whole words, and triple-click-and-drag by whole lines — in both TextEdit and labels, including selections spanning multiple labels.

The reason this was missing: clicks are only counted on pointer release, so at the start of a drag nothing knew it began with a double-click.

• PointerState now also computes the would-be click count at press time (pub(crate) only, same heuristics as the click counting on release).
• On a second/third press, TextCursorState selects the word/line under the pointer immediately and remembers it as the anchor; while dragging, the selection is the union of the anchor and the word/line under the pointer. A side effect is that plain double-click selection now appears on press rather than release, matching other UI toolkits.
• LabelSelectionState carries the anchor across passes, so the drag can extend into neighboring labels.

No public API changes. Covered by unit tests for the range math and kittest tests for TextEdit (forward/backward/lines) and cross-label drags in tests/egui_tests/tests/test_text_selection.rs.

[github-actions]

Preview is being built...

Preview will be available at https://egui-pr-preview.github.io/pr/8370-word-drag-selection

View snapshot changes at kitdiff