Initial support for grid stitching from paths

This PR is a preliminary work towards the stitching use case described in imagej/imagej-omero#45.

The intent would be to enable file lookup from a list of absolute paths in arbitrary directories (such as the OMERO ManagedRepository), without duplication to a temporary location beforehand and with moderate refactoring impact on the plugin.
Ideally, all grid types exposed in the plugin would be candidates for such support, but the current patch is restricted to a minimal subset in order to get a prototype running and gather early feedback on this approach.

The current behaviour is as follows:
* limited to _Unknown positions_ and _Sequential Images_ grid types
* lookup the paths to tiles from a _TilePath.txt_ file:
  - the file may be located within the same directory as _TileConfiguration.txt_ or elsewhere (side note: paths read from/written to _TileConfiguration.txt_ are currently unchanged)
  - the file content is expected to be a list of absolute paths (one per line - comments (#), empty lines and relative paths are ignored) - eg.
```
# Ignore the first three lines
ignore_relative_path.tif

/path/to/tile_1.tif
/path/to/tile_i.tif
```