Add tests for FS

[ice0]

          > We have to test a lot of those path now :(

I'll list the needed tests later too

Tests related to files that we must do (specially on Windows with Cyrillic/Arabic as system 'locale'):

Sketch

• Save a sketch with a pure ASCII file path
• Save a sketch with a basename with non latin characters
• Save a sketch on a directory with non latin characters
• Load a sketch with a pure ASCII file path
• Load a sketch from a basename with non latin characters
• Load a sketch from a directory with non latin characters

Palette

• Save a palette with a pure ASCII file path
• Save a palette with a basename with non latin characters
• Save a palette on a directory with non latin characters
• Load a palette with a pure ASCII file path
• Load a palette from a basename with non latin characters
• Load a palette from a directory with non latin characters

Settings folder

Synfig settings path ([user-folder]/.config/synfig folder or that from SYNFIG_USER_SETTINGS environment variable):

• Save workspaces with Synfig settings path with a pure ASCII file path
• Load workspaces with Synfig settings path that contains non latin characters
  .
• Save general settings with Synfig settings path with a pure ASCII file path
• Load general settings with Synfig settings path that contains non latin characters

Import via File > Import

• Import file with a pure ASCII file path to a NON-saved Synfig document
• Import file with a basename with non latin characters to a NON-saved Synfig document
• Import file from a directory with non latin characters to a NON-saved Synfig document
• Import file from a different drive (on MS Windows only) to a NON-saved Synfig document
  .
• Import file with a pure ASCII file path to a saved Synfig document
• Import file with a basename with non latin characters to a saved Synfig document
• Import file from a directory with non latin characters to a saved Synfig document
• Import file from a different drive (on MS Windows only) to a saved Synfig document (Absolute paths don't work anymore #3212)

Import sequence via File > Import sequence

Import via drag and drop on CanvasView (#3279)

Open via File > Open

Open via drag and drop on Tool panel

Open via Recent file menu

Open via system (double click on .sif(z) document or prompt)

Sound Layer

Sound panel

Text Layer

Brushes path

Save

Export

Plugins

• run plugin on an unsaved file
• run plugin on a saved file with pure latin characters on file path
• run plugin on a saved file with non latin characters on file path
• run plugin whose installation folder has non latin character on file path

Render

Monitored files

Originally posted by @rodolforg in #3279 (comment)