Animation Render minor bug fixes and usability improvements

A few bug fixes, though I am not sure the "Make FFMpeg render warn on file selection " is a bugfix or a usability improvement, so I didn't label it as bugfix.

---

Bugfix: Make APNG format for export consistent

This patch makes apng the default format in the filename textbox for export instead of png, making it consistent with the file dialog.

Fixes BUG #451473

Bugfix: Do not label mp4 unsupported when None

On first use, when someone open up the render animation window, it tells them they do not have FFMpeg and also says it does not support MKV/MP4. This patch fixes the confusion and just labels it as None. MP4/MKV unsupported will only now show when it truely isn't supported by FFmpeg.

Make FFMpeg render warn on file selection

When a user selects an improper FFMpeg, they only see None indicator, but do not know what the issue is until actually hitting render. This can be a bit confusing, especially for those who forgot to extract the archive.

This patch makes it so when a user selects a file, it will give the user a proper message when selected. It also disables the user from manually typing in the FFMpeg path into the textbox, and all entry has to now be through the file dialog. This makes it more consistant with recorder and import animation, stop checking version every keystroke and remove room for error. Lastly, this patch adds 7z as possible archive type to warn about extracting.

Test Plan

Bugfix: Make APNG format for export consistent

1. Make or open an animation and select Animated PNG, it should end with .apng format

Bugfix: Do not label mp4 unsupported when None

1. If you are on linux or using a non-static build of ffmpeg, either uninstall ffmpeg, or temporary rename it to ffmpeg2 (This is cause findffmpeg will default to the OS installed version if a wrong version is picked)
2. Open up any file that isn't ffmpeg so that it says None. It should not say that MP4/MKV is unsupported
3. Now open an LGPL build of FFMpeg, it should say it is unsupported.
4. Don't forget to restore your old version of ffmpeg if you did so in the 1st step

Make FFMpeg render warn on file selection

1. Try editing the ffmpeg path in the textbox, it should not let you
2. Try adding any file as ffmpeg, it should give an error telling you that it isn't a valid ffmpeg
3. Try adding a 7z file, it will tell you to extract the archive

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.