applets/comic: Port to KF6/Qt6

Now it's comics' turn:

• Remove usage of PlasmaExtras (it only used ScrollArea, so it was easy enough to change it to ScrollView)
• Replace usage of iconSource
• Port to PC3, it used the tabbar from PC2 still
• Remove some deprecated Qt5 code, which we don't have to support anymore

Of note, .desktop metadata files aren't supported anymore so no previous comic plugins will show up. For testing, I have created a functional metadata.json replacmenet for the bideja plugin:

{
    "KPlugin": {
        "Authors": [
            {
                "Email": "oluisedu@gmail.com",
                "Name": "Luiç"
            }
        ],
        "Id": "bideja",
        "Name": "Bideja",
        "Version": "1.0",
        "Icon": "icon.png"
    },
    "X-KDE-PlasmaComicProvider-SuffixType": "string",
    "KPackageStructure": "Plasma/Comic",
    "X-KDE-Library": "plasma_comic_krossprovider"
}