Enable localized news on the welcome page

Add the ability to show localized news on the welcome page. Currently, these languages are hard-coded in:

• en -> English
• jp -> 日本語 - Japanese
• zh -> 中文 (简体) - Chinese, Simplified
• zh-tw -> 中文 (台灣正體) - Chinese, Taiwan Traditional
• zh-hk -> 香港廣東話 - Hong Kong Cantonese

The user is allowed to select multiple languages for now. The feeds will be shown together, sorted by date/time.

https://bugs.kde.org/show_bug.cgi?id=427640

TODO

• Add code to load/save the selected languages
• Add default language selection according to primary UI language
• Decide whether the "Language" button is good UX, should the style be changed, etc
• Find a way to make a note that "English news is always up to date" to let the user know that localized news may become out of date
• Fix the repaint and layout issues?

Test Plan

Change the selected languages from the "Language" dropdown in the "News" section and note that the news items changes.

Note: It appears that the news may not be immediately repainted when removing a feed (i.e. disabling a language).

Formalities Checklist

• I confirmed this builds.
• I confirmed Krita ran and the relevant functions work.
• I tested the relevant unit tests and can confirm they are not broken. (If not possible, don't hesitate to ask for help!)
• I made sure my commits build individually and have good descriptions as per KDE guidelines.
• I made sure my code conforms to the standards set in the HACKING file.
• I can confirm the code is licensed and attributed appropriately, and that unattributed code is mine, as per KDE Licensing Policy.