Refactored PR: Theming of disabled knobs

[thmueller64]

This PR partly addresses #4749 and introduces the possibility to theme disabled knobs. I refactored my old draft PR #5481, based on suggestions by @Veratil, using a similar approach as #5253.

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Linux

• Linux (AppImage): lmms-1.2.2.698-linux-x86_64.AppImage (build link)

Windows

• Windows 32-bit: lmms-1.2.2.698-mingw-win32.exe (build link)
• Windows 64-bit: lmms-1.2.2.698-mingw-win64.exe (build link)
• Windows 32-bit: lmms-1.2.2-msvc2017-win32.exe (build link)
• Windows 64-bit: lmms-1.2.2-msvc2017-win64.exe (build link)

macOS

• macOS : lmms-1.2.2.698-mac10.13.dmg (build link)

🤖

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[Veratil]

One thing that might be cool to do and allow even cooler themes is to allow gradients instead of single colors. Changing these to QBrush would allow you to define gradients or colors.

[thmueller64]

Thanks @Veratil for the suggestion. I replaced QColor by QBrush, below is an example. You get the idea from the picture, but a QLinearGradient does not result in the intended behavior here, see e.g. the red color at low volumes (I think this is due to the gradient being applied independent of the length of the active line). Furthermore, you can observe that m_lineColor is used both for the "middle line" and the active part of the "arc line".
Maybe we could address this feature separately?

[Veratil]

You wouldn't use a linear gradient in that instance, you'd use a conical gradient in the CSS. Example from Qt documentation:

/* conical gradient from white to green */
QTextEdit {
    background: qconicalgradient(cx:0.5, cy:0.5, angle:30,
                stop:0 white, stop:1 #00FF00)
}

At the same time though, if you wanted single color you can define the brush to be the normal rgba(...).

[PhysSong]

Can we merge this once the open discussions are resolved?

[zonkmachine]

I think this works really well now.

[Veratil]

@thmueller64 Does this need an update the classic theme as well?

Other than my suggestion for using QBrush instead of QColor, LGTM. 👍

[zonkmachine]

This is what the classic theme looks like right now. Just black lines, could be improved.

[thmueller64]

@thmueller64 Does this need an update the classic theme as well?

Other than my suggestion for using QBrush instead of QColor, LGTM. +1

I updated the classic theme. Thanks again for the suggestion. I've looked into the conical gradient, but didn't find a straightforward way to replace QColor this way. For example, the middle line is drawn in the same color as the active part of the arc, so I would possibly have to introduce a separate color for it. I suggest to address this in another PR.

[DomClark]

It might be a good idea to explicitly #include <QPixmap> - it's currently available via <QWidget>, but if this is changed to use a forward declaration, the unique_ptr here will fail to compile since QPixmap's destructor won't be declared.

[DomClark]

LGTM

[zonkmachine]

@thmueller64 Looks good! Ready to merge?

[zonkmachine]

This PR partly addresses #4749

It looks like this fixes #4749 completely. What's left to do?

I think this should be merged.