qmllint shipped with Qt 6.12 segfaults on the inline component
definition inside RunningAppsPanel.qml, breaking the
suse_tumbleweed_qt612 KDE Invent CI job. Move PanelIconButton out
to a sibling RunningAppsPanelButton.qml so qmllint never has to
parse a UiInlineComponent here.
The extracted file inlines its own colour-mix helper to keep it
self-contained and avoid leaking RunningAppsPanel internals.
