Fix problems with relative paths on Windows

[w1th0utnam3]

On Windows, sequence loading with relative paths were broken (not sure when it worked for the last time). More specifically:

• fileseq.findSequencesOnDisk cannot deal with Blender relative paths (starting with //) and needs a kind-of absolute path. Solution: use bpy.path.abspath
• The same applies to fileseq.findSequenceOnDisk, however this also seems to break with paths containing ... Solution: convert to a fully absolute path with os.path.abspath
• Extracting a sequence directory and pattern with os.path.dirname/basename does not work on Blender relative paths. Solution: do this before converting absolute to relative paths
• At least on Windows, bpy.path.relpath and bpy.path.abspath do not work correctly with a Blender relative path as start argument. Solution: turn it into an absolute path with bpy.path.abspath

I tested these changes on MacOS and they seem to work as intended.

Also this PR changes the recursive load to include the selected folder itself which is more in line with the common understanding of recursive commands. Alternatively, we could change the name to something like "load from subfolders recursively" or something like this.