[FEATURE] Improve UI/UX on Three-Way Merge Editor

[LennartCode]

Currently the three-way merge editor is not very user friendly. Especially people with no git experience might have serious trouble understanding it.

Here are some suggestions:

• Actually do three-way merging using a base file, for example from Syncthing's history (similarly described here) instead of a two way merge with displayed output
• Add an option to either display the result as rendered markdown or plain text
• Correctly render the resulting markdown (for example relevant for frontmatter) or detect the problem, choose plain text and display an explanation (See [BUG] Markdown is not rendered correctly in Preview Merged File #215)
• Open the merge editor directly instead of the Difference View when selecting view conflict
• Provide buttons to navigate to the next conflict in a footer or header
• Hide the meta data behind a button, menu or such. In most cases everything but the file name is irrelevant to the user
• In the three-way merge editor provide buttons to accept either of the files completely
• Use cherry-picking for constructing the merged file, similarly to what VSCode does but label the files left and right or after the devices instead of incoming and current as the user does not need to have a concept of incoming changes to successfully merge and this might increase confusion

These suggestions are rather opinionated and mostly stand on their own. I would like to inspire changes and am happy to discuss them!

[LBF38]

Thank you for your feedback ! I'll include them in future versions when I've got time.
Feel free to contribute to the project if you want too.