python3 port: use print() as function
python3 port: various syntax fixes (exceptions, has_key(), range())
python3 port: don't use separate compare function for sort
remove ProjectMarker plugin support

The ProjectMarker plugin stores the project dir as "data" for each document,
but the get_data() function is apparently not supported by new Python/GTK
bindings any more. As the plugin doesn't seem to be maintained any more,
just remove this feature until somebody yells.
search_dialog.py: some code simplifications
search_dialog.py: some more straightforward code simplifications
remove any uses of obsolete GTK get_data()/set_data() API
python3 port: don't use dircache module any more

Unfortunately this makes directory completion noticeably slower eg. for
badly-connected GVFS shares, probably due to the additional isdir() calls.
I guess a better solution would be to use asynchronous GIO functions
instead.
python3 port: cope with new urllib module structure (for quote/unquote)
python3 port: use new syntax for relative module imports
python3 port: use Unicode strings everywhere

This code should work under Python 2 and 3.

Unfortunately GTK always returns "str" objects, which have different
meanings under Python 2 and 3. The new gtkToUnicode() function now helps to
always get Unicode strings from GTK widgets.

Note: the code in searcher.py is now worse than before, because it now
assumes that output from grep and find is always valid UTF-8. This is not
necessarily the case (file names and file contents can be in a different
encoding or even have no sane encoding at all). I guess lots of code must
be changed yet to use raw byte strings instead.
fix issue #15: README file now contains download link for version 1.0
python3 port: add __pycache__ dir to gitignore