Add ability to see main members in groups

[TGODiamond]

Adds the ability to see main members in groups. (Inverse of a user's main group)

Also fixes the translation file lxqt-admin-user_kab.ts
Edit: This can be done by @stefonarch instead.

As Im new to C++, I just want to make sure this is safe and good:

// add groups to this user and add this user to their group's main members
for(GroupInfo* group: mGroups) { // Removed the "std::as_const(mGroups)", replaced with "mGroups"
    if(group->hasMember(user->name()))
        user->addGroup(group->name());
    if(user->gid() == group->gid())
        group->addMainMember(user->name());
}

Screenshots

When adding a new group:

[tsujan]

Please remove the changes to translations from your PR.

[TGODiamond]

Oh, are they automated?

[TGODiamond]

It is mostly correcting the line numbers, and adding the new, missing entries.

[tsujan]

Translation updates are done separately, by @stefonarch . Code PRs shouldn't touch translation files.

[TGODiamond]

Done!

[tsujan]

Thanks for the PR and also for cleaning it up. I requested a review.

[palinek]

Sorry, but I don't see a benefit of having visually separated list of main member. Wouldn't it be enough just to assure, that the main member is always first in the list of members?

[TGODiamond]

There can be more than one main member in a group.

[palinek]

Sure, and what is the purpose of showing them in GUI separated list?
Again wouldn't it be enough just to put the main members as first in the list of standard members?

[TGODiamond]

Users can both be a main member and a normal member of a group at the same time.

How should that be shown in the group GUI?

However, I find it understandable to show both normal and main members in the same column in the group list GUI.

[palinek]

How should that be shown in the group GUI?

First in the list with disabled checkbox?

[TGODiamond]

Then there will be both the main members with checkbox disabled and and normal members with selectable checkboxes, which will ALWAYS include the main members as normal users, since they must also be controlled if they are normal members or not.

This means that there will 2 entries for every member, if they are a main member, which will be confusing, especially for new people to this program.

[TGODiamond]

I could move the main members list to be over the normal members, and make the main members list thin.