[gui] sync layer right-click menu & composer legend item icons

[nirvn]

This pull request does the following:

• update the composer legend item panel's filter button
• fix size of the composer legend item panel's add group button and make use of refreshed icon
• update the layer panel's right-click menu items to make use of refreshed icons
• add a [+] green box to the layer panel's add group button (this harmonizes the action icon for the composer legend item and the layer panel)

[nirvn]

While we're at it, I've updated the duplicate layer icon to align to a 16 x 16 grid to remove the blurry rendering within the layer item right-click menu. It's the sole place where it's used, so we should aim at looking good there :)

[nirvn]

A couple of before and after shots: