Change symbols color

[rysebaert]

In the layers panel, make possible to change color of displayed symbols, as proposed for categorical maps.

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[mthh]

Thanks !
Ideally, this should be done while solving issue #69 (which proposes, among other things, to allow changing the background and outline colors of symbols added individually to the map).

[mthh]

Turns out that #69 and #76 require a little more work than I expected 😓

[mthh]

Symbols in the new version can be (to some extent) customized in order to change their color or border. But there's still room for improvement here.