#2 Large files handling

[alsemm]

Editor should support load/save fragments of text to file system. When large file is to be loaded in editor it shouldn't try to load entire file contents in memory. Instead editor should break the file into segments (original file shouldn't be modified) and load only those segments that are needed for display or modification. Editor also should keep track of memory used. If it exceeds some configurable limit editor should purge some text data to the file system. "unsigned" type doesn't work for character positions/line index with large files. So public API should be changed.