Switch sender colors to use color palette

Switch sender colors to use color palette

Add "sender-color-[hash]" to the list of valid palette color roles.

Switch QtUiStyle::generateSettingsQss() to make use of the color
palette, rather than redundantly specifying the color.  This cleans
up the output of settings.qss, and (more importantly) allows custom
themes to do the same.

There's trade-offs involved with using the color palette directly
(similar to MarkerLine).  It would make the GUI capable of toggling
sender coloring in themes, which would be nice.  However, themes
would no longer be able to set sender colors independently for
different message types.

Though untested, there might be performance trade-offs with querying
the settings provider on each render, too.

This can be revisited in the future.