Theming of disabled knobs

[thmueller64]

This PR partly addresses #4749 and introduces the possibility to theme disabled knobs.
I could not get it to work using the pseudo state ":disabled" in the style sheets, as suggested in #4749. As the knob icons are png Images, we somehow have to catch the enabledChange event and convert the image to grayscale. Moreover, the pseudo-states do not seem to work with a custom QPROPERTY, in contrast to the standard ones like background-color etc. . So redrawing the middle line and arc does not seem possible with pseudo-states.

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Linux

• Linux (AppImage): lmms-1.2.1.648-linux-x86_64.AppImage (build link)

Windows

• Windows 32-bit: lmms-1.2.1.648-mingw-win32.exe (build link)
• Windows 64-bit: lmms-1.2.1.648-mingw-win64.exe (build link)
• Windows 32-bit: lmms-1.2.1-msvc2017-win32.exe (build link)
• Windows 64-bit: lmms-1.2.1-msvc2017-win64.exe (build link)

macOS

• macOS : lmms-1.2.1.648-mac10.13.dmg (build link)

🤖

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[PhysSong]

@thmueller64 Can we close this in favor of #5549?

[thmueller64]

Yes, I've forgot to close this one, sorry!