Adds local ignoring, i.e., Exlude file ignoring

[abc35277]

Often, we have a need to locally need these files or folders but don't want to upload them to Git, such as publishing folders, local AI folders, etc. You need to right-click on the UI to support local ignoring them.

[love-linger]

SourceGit only supports to ignore untracked files.

When a file has been already added to the repository, this context menu entry will not exist

[love-linger]

For folders, there are always a context menu entry Ignore untracked files in this folder