Emit slider's drag_started signal before the first value change

[timothyqiu]

Fixes #86370

#80916 made value_changed to emit after drag_started in order to fix ColorPicker's deferred mode. But the actual value change is still before drag_started.

It was then impossible to safely ignore value_changed signal when dragging, because you'll miss the value_changed signal when pressing down the mouse button.

This PR:

1. Moves drag_started before the initial value change, so users can get the correct start value when handling the signal.
2. Records the value before the initial value change, so that drag_ended can correctly tell that the value is changed if the user simply clicks somewhere on the slider other than the grabber.

[YuriSizov]

Thanks!