Abilitity to load list of files from external text file

Currently the list of files to be worked on is generated by the three fields "Pattern", "recursive" and hidden. Sometimes the list of files is more complex than what can be generated by this command. For such a cases it would be nice if the list of files can be generated externally and then loaded into regexxer.

I suggest the following changes:

1. Add menu entry File/Load file list that calls action load_file_list
2. In load_file_list open up a GtkFileChooserDialog.
3. Create a callback for accepting a list filename.
4. Foreach line in the file check if it is an existing filename (this should preferably be relative to the list filename).
5. Add the basename of the filename to filelist widget.

This interface will make the FindFiles input inconsistent with the contents of the file list, but that is anyhow the case right now since the Pattern entry may be edited without pressing the Find files button.