Further SampleEditor tweaks

[theGreatWhiteShark]

• [ ] Open the SampleEditor -> enter a large value in the spinbox of the "Start" and press enter -> both the values in the spinboxes of "Start" and "Loop" get updated but only the "S" line gets moved in the wave display while "L" remains at zero.
• [ ] Open the SampleEditor -> change something and hit the "Close" button -> a dialog "Unsave changes..." will pop up and the user has to consent to discard the changes -> clicking "Ok" and reopen the SampleEditor by clicking "Edit Layer" again -> the same popup shows up again. I think the second one is a dud.
• [ ] When entering a fresh instance of the SampleEditor the value of the EndFrameSpinBox looks perfectly fine. But as soon as the mouse pointer enters the MainSampleWaveDisplay it is reset to 0 by SampleEditor::getAllFrameInfos() what causes some other oddities. The slider, however, stays at it's correct position. This also caused #811.
• [ ] The first value change in the Start, Loop, and End spinboxes is discarded and, instead, the slider is shown in the DetailWaveDisplay. It would be nice to update the DetailWaveDisplay as soon as the spinboxes have focus and make the first change affect the value as well.
• [ ] When both the volume and pan envelops are edited, it's not straight forward to see which one is selected by looking at the envelopes. If think it would improve usability to make the only currently not selected more transparent to indicate its state.
• [ ] When entering SampleEditor playback is still rolling but can not be started/stopped till the dialog is closed again. That's not good. Either it is a proper modal and transport stops or no modal and transport keeps rolling.
• [ ] In case transport keeps rolling: playback of the sample currently edited produces audible artifacts as soon as changes are applied.

[charbeljc]

* Open the `SampleEditor` -> enter a large value in the spinbox of the "Start" and press enter -> both the values in the spinboxes of "Start" and "Loop" get updated but only the "S" line gets moved in the wave display while "L" remains at zero.

Hum, I'm afraid this one was my fault ..

* Open the `SampleEditor` -> change something and hit the "Close" button -> a dialog "Unsave changes..." will pop up and the user has to consent to discard the changes -> clicking "Ok" and reopen the `SampleEditor` by clicking "Edit Layer" again -> the same popup shows up again. I think the second one is a dud.

Same here, the call to setClean() was misplaced in the SampleEditor's setup.

* When entering a fresh instance of the `SampleEditor` the value of the `EndFrameSpinBox` looks perfectly fine. But as soon as the mouse pointer enters the `MainSampleWaveDisplay` it is reset to 0 by `SampleEditor::getAllFrameInfos()` what causes some other oddities. The slider, however, stays at it's correct position. This also caused [sample editor : end marker jumps to start marker position #811](https://github.com/hydrogen-music/hydrogen/issues/811).

This is fixed with my PR.

* The first value change in the Start, Loop, and End spinboxes is discarded and, instead, the slider is shown in the `DetailWaveDisplay`. It would be nice to update the `DetailWaveDisplay` as soon as the spinboxes have focus and make the first change affect the value as well.

Also done in #1314, Regards.

[theGreatWhiteShark]

This is fixed with my PR.

Nice! I will have a look tomorrow.