Popup for configuring movement speed when moving edit camera with keyboard

If entire scene is in very large or very small scale, or if you want to fly around zoomed into very small part of the scene, the default movement amounts of WASD do not make sense, being either too large or too small. 

A toolbar button should be added next to existing align camera controls in 3D view, which opens a configuration popup for the camera speed. It should have a slider and a scaler field.