Show interactive placeholder buttons in the video and waveforms area

[nekohayo]

The video player and waveforms area aren't as welcoming as they could be. They could have a bit of informative text and buttons like this, instead of just leaving the space empty:

|----------------------------------|------------------------------------------------|
| No video file specified.         | No waveforms present. 
| Drag-and-drop one here           | Generate one from the video file
| or browse with the button below. | or load a previously generated one.
|                                  |
|     ( Open video file... )       |  ( Generate waveforms... ) ( Open .wf file... )