vc: Fix "meld <file>" operation

Add a check to choose_vc() to make sure it always checks if a directory
supports version control.  Previously it would check if a file supported
version control.  Version control is always supported on a directory
basis, and the underlying meld version control support doesn't support
checking if a specific file is under version control which would lead to
a runtime error similar to:

Traceback (most recent call last):
  File "/home/user/meld/meld/meldapp.py", line 81, in do_command_line
    self.parse_args(command_line.get_arguments()[1:])
  File "/home/user/meld/meld/meldapp.py", line 205, in parse_args
    auto_merge=options.auto_merge, new_tab=options.newtab)
  File "/home/user/meld/meld/meldapp.py", line 120, in open_paths
    return self.window.open_paths(paths, **kwargs)
  File "/home/user/meld/meld/meldwindow.py", line 713, in open_paths
    self._single_file_open(a)
  File "/home/user/meld/meld/meldwindow.py", line 703, in _single_file_open
    doc.set_location(path)
  File "/home/user/meld/NEWS", line 356, in set_location
    self.choose_vc(location)
  File "/home/user/meld/NEWS", line 304, in choose_vc
    elif not avc.valid_repo(location):
  File "/home/user/meld/meld/vc/git.py", line 263, in valid_repo
    return not _vc.call([cls.CMD, "branch"], cwd=path)
  File "/home/user/meld/meld/vc/_vc.py", line 338, in call
    return subprocess.call(cmd, cwd=cwd, stdout=NULL, stderr=NULL)
  File "/usr/lib/python2.7/subprocess.py", line 522, in call
    return Popen(*popenargs, **kwargs).wait()
  File "/usr/lib/python2.7/subprocess.py", line 709, in __init__
    errread, errwrite)
  File "/usr/lib/python2.7/subprocess.py", line 1326, in _execute_child
    raise child_exception
OSError: [Errno 20] Not a directory: '/home/user/meld/NEWS'

Signed-off-by: Peter Tyser <ptyser@gmail.com>
findbar: Convert from GtkTable to GtkGrid

Signed-off-by: Peter Tyser <ptyser@gmail.com>
filediff: Convert from GtkTable to GtkGrid

Signed-off-by: Peter Tyser <ptyser@gmail.com>
dirdiff: Convert from GtkTable to GtkGrid

Signed-off-by: Peter Tyser <ptyser@gmail.com>
tab-placeholder: Convert from GtkTable to GtkGrid

Signed-off-by: Peter Tyser <ptyser@gmail.com>
dirdiff, vcview: Update GtkTreePath handling to avoid using tuples
vcview.ui: Fix accelerator on commit dialog button
dirdiff.ui: Resave with modern glade and update expand flags