Change appeareance of buttons according to the state of the interface

For now, all the buttons for uploading and deleting both audio and markers files in both first and second signal panels have the same appearance, regardless of the fact that they are active or not. When the frontend is started, only the upload button (currently with the clip image) for the audio signals should be active. This must be visually indicated to the user. For example, the inactive buttons should be grayed out and, most importantly, should not react to mouse hovering.

Once the audio file is uploaded, then the delete button (currently with the garbage can image) for the audio file and the upload button for the markers file must become active. Once the markers file is uploaded, then the delete button for it must become active.