This commit introduces a context menu for Tab and a new item,
"Color...", which will display a color picker.
A flyout menu, containing a custom flyout, is attached to each tab. The
flyout displays a palette of 16 preset colors and includes a color
picker. When the user selects or clears color, an event is fired, which
is intercepted by the tab to which the flyout belongs.
The changing of the color is achieved by putting the selected color in
the resource dictionary of the tab, using well-defined dictionary keys
(e.g. TabViewItemHeaderBackground). Afterwards the visual state of the
tab is toggled, so that the color change is visible immediately.
Custom-colored tabs will be desaturated (somewhat) by alpha blending
them with the tab bar background.
The flyout menu also contains a 'Close' flyout item.
## Validation Steps Performed
I've validated the behavior manually: start the program via the start
menu. Right click on the tab -> Choose a tab color.
The color flyout is going to be shown. Click a color swatch or click
'Select a custom color' to use the color picker. Use the 'Clear the
current color' to remove the custom color.
Closes#2994. References #3327.
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