Android animation export

This implements animation export on Android and some ancillary things that came along with it.

• GIF rendering through libav. WEBP and APNG exporting also exists in theory, but we don't build libav with those encoders.
• Animation video exporting, including audio. Didn't work at all before.
• Animation image sequence exporting. Barely worked before, spewed errors when copying identical frames.
• Warnings when specifying a framerate above 30 or dimensions above 1920 pixels. These often fail to export or play back, not just on Android, so the warnings exist on all platforms now. That warning message also now has a proper icon and is less jerky when updating.
• Using the "next" icon for the app in debug builds, since that's also what gets used inside of Krita.

KA thread: https://krita-artists.org/t/testing-wanted-android-animation-export/186834

Test Plan

See the KA thread.

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.
• Does the patch add a user-visible feature? If yes, is there a documentation MR ready for it at Krita Documentation Repository?

Reminder: the reviewer is responsible for merging the patch, this is to ensure at the least two people can build the patch. In case a patch breaks the build, both the author and the reviewer should be contacted to fix the build. If this is not possible, the commits shall be reverted, and a notification with the reasoning and any relevant logs shall be sent to the mailing list, kimageshop@kde.org.