Skins: append / when setting skin base path

[ronso0]

This ensures stylesheet icon urls without leading / are transformed to correct paths.

Regression from #3877, noticed with DarkMetal https://mixxx.discourse.group/t/crates-playlists-history-etc-vanish-from-dark-metal/22704/ : the user assumed the library was broken because the sidebar expand arrow icons were missing...

So my hasty conclusion was wrong

mixxx/src/skin/legacy/legacyskinparser.cpp

Lines 2311 to 2313 in 1557ef2

	// Note: It's safe to use the base path after parseSkin() has updated it
	// (parseLaunchImage() appends "/" earlier)
	return style.replace("url(skin:", "url(" + m_pContext->getSkinBasePath());

because the skin base path is also set by LegacySkinParser::parseSkin() without trailing /

Appending the / will result in // but this is filtered by Qt later on (assuming since leading / are used for the LateNight launch image and that works on all platforms)

Proposed Changelog entry

- Fix support for relative paths in the skin system which caused missing images in third-party skins. #4151

[uklotzde]

If it works, then let's use this simple fix.

[Holzhaus]

LGTM, thanks.