Add a hotkey to select all keys in a timeline

Currently, selecting all keys in a specific timeline from the first key to the last always requires using box selection.

There is already a `Select Keys` hotkey for selecting keys on the current frame, but there is no shortcut for selecting all keys within a single timeline.

Adding a hotkey for this could make timeline editing more efficient, especially when users frequently need to operate on all keys of a particular property.

**Related forum thread:**
https://esotericsoftware.com/forum/d/30406-%E6%9C%89%E5%85%B3%E6%91%84%E5%BD%B1%E8%A1%A8%E5%9B%BE%E8%A1%A8%E7%9B%B8%E5%85%B3%E7%89%B9%E6%80%A7%E5%92%8C%E5%8A%9F%E8%83%BD%E5%BB%BA%E8%AE%AE

Start by locating the existing `Select Keys` hotkey and the timeline key-selection entry point. Define a shortcut for selecting every key in the current timeline, then verify that it selects from the first key through the last without requiring box selection.

Written by the indexing model from the issue text.