Base path for local library

[MathemanFlo]

It would be great if ArmorPaint had an option to set a base path for a library that contains materials, brushes, textures, swatches, ... The option could be made in two ways:

1. Set a base path in the preferences. Therefore it is for all projects. This is probably the simpler the solution.
2. Set a base path on a per project basis. This would allow the user to have multiple libraries.

The behavior could be the following:

1. The user loads a texture from a path that is a child of the base path. In this case ArmorPaint stores the path without the library prefix. I.e. if the user saves the project, closes ArmorPaint, moves the library to a different location, opens ArmorPaint again, changes the base path, loads the project again, everything will be fine.
2. The user loads a texture from another path. Currently the path is stored relative to the Project file. On the one hand this is good because it allows the user to move the whole directory tree to a different location which is great, too. On the other hand it is impossible to move the project file without breaking the project. So I have no definite opinion on that.