Implement system tray color detection for KDE

[dragotin]

The icon of the OpenCloud sync client appears to be very colorless and pale in the side panel of KDE.

@tbsbdr

[tbsbdr]

Violates a11y Success Criterion 1.4.3 Contrast (Minimum) https://www.w3.org/TR/WCAG22/#contrast-minimum

[TheOneRing]

Apparently, there is no proper way to detect the systray colour of KDE plasma.

Using a mask icon as we do on mac also had no effect.