Minor LookAndFeel_V3 request (use of a colourId)

General JUCE discussion
1

At the moment, lnf3 does not use TabbedButtonBar::frontOutlineColourId.
The following small changes would avoid the need of overriding the full methods just to have it.

In the constructor (to have the same colour as the TabbedButtonBar::tabOutlineColourId one) :

setColour (TabbedButtonBar::frontOutlineColourId, Colour (0x66000000));

in LookAndFeel_V3::drawTabAreaBehindFrontButton() :

if (button.isFrontTab())
    g.setColour (bar.findColour (TabbedButtonBar::frontOutlineColourId));
else
    g.setColour (bar.findColour (TabbedButtonBar::tabOutlineColourId));

in LookAndFeel_V3::drawTabButton() :

if (button.isFrontTab())
    g.setColour (button.findColour (TabbedButtonBar::frontOutlineColourId));
else
    g.setColour (button.findColour (TabbedButtonBar::tabOutlineColourId));