Show color usage and warn on spot color deletion

[dl3sdo]

When a user wants to delete a color, the warning message was ambiguous by mixing up symbols and objects (see #2127).
This PR merges #2127, #2222, #2209.
It shows one warning dialog if

• the color is used by symbols
• the color is used as a spot color by other colors.

In addition the warning messages specifies the number of symbols, objects and other map colors that will be affected by the color removal.
A more detailed warning is available when removing spot colors.

[dl3sdo]

[dl3sdo]

Kai, Libor: What do you think? Does this PR meet your functional expectations?

[dg0yt]

I couldn't look at it in detail.

• There is a lot of extra code in a single GUI class method which might be better separated into non-GUI functions with clear interfaces.
• The translatable strings must form whole sentences or paragraphs. My suggestion:
  • General message:

    tr("Color \"%1\" is used by other elements. Removing the color will change the appearance of these elements. Do you really want to remove it?").arg(color_to_be_removed->getName());

  • Details:

    if (<used_by_colors>)
    {
        tr("This color is used as a spot color by the following map colors:")
        <list>
    }
    if (<used_by_symbols>)
    {
        tr("This color is used by the following symbols:")
        <list of symbol_name: n objects>
    }

[dl3sdo]

@dg0yt: Following your suggestions:

[dg0yt]

Integrated and merged with #2222. Tnx!