!F Adding gifski gif converter

[pr8x]

This is a working draft of integrating gifski into Captura. Things to consider:

• I have not yet implemented GifskiOptions. The exe path is hardcoded at the moment.
• In my experiments the produced files by gifski are actually quite a lot larger than the ones produced with the ffmpeg gif converter. We may want to play with the --quality and --fps option and/or downsample resolution (suboptimal). The USP of gifski is quality not compression, so we shouldn't expect too much.
• In general I feel like parts of the code base could use some linting. Namespaces are quite inconsistent (Example: Some classes in the ffmpeg implementation use Captura.FFmpeg, most use Captura.Models). I think that should be fixed. Ideally, they should reside in a namespace named like the project/folder IMO.

Fixed issues: #457

[MathewSachin]

Looks good so far.
The namespace thing was what I wanted to do but put off due to other things. But, doing that for every folder might be an overkill. Namespaces as per the project name should be enough I think.

[pr8x]

Hi I am currently quite busy with work so If anybody wants to actually fork this and add some option to download gifski in the settings... That would be cool!