qtutils: remove unused functions

The icon_for_file() function and its only caller update_file_icons()
were unused so remove them.

Signed-off-by: Daniel Harding <dharding@living180.net>
qtutils: rename mimetypes() to path_mimetypes()

This will enable some later code refactoring by allowing Python's
mimetypes module to be imported in the cola.qtutils module without
a name collision.

Signed-off-by: Daniel Harding <dharding@living180.net>
utils: refactor ident_file_type()

* Rename KNOWN_FILE_EXTENSION to KNOWN_FILE_EXTENSIONS.
* KNOWN_FILE_MIME_TYPES was never used for dictionary lookups (it was
  only iterated over), so convert it to a list of two-element tuples.
* Convert the guessed mimetype to lowercase once instead of every time
  through the loop of KNOWN_FILE_MIME_TYPES.
* Look up the filename's extension in the KNOWN_FILE_EXTENSIONS
  dictionary instead of iterating over it and testing with .endswith().

Signed-off-by: Daniel Harding <dharding@living180.net>
qtutils: move icon functionality from utils

The utils.file_icon() and supporting utils.ident_file_type() functions
did not really belong in the utils module because they have to do with
the QT UI.  The utils.file_icon() function was only used in one place,
in the qtutils.icon_file() function, so inline its functionality there,
and move ident_file_type() from utils to qtutils.

Signed-off-by: Daniel Harding <dharding@living180.net>
qtutils: reduce calls to resources.icon()

Change some of the icon functions to return icon names that get passed
to a single call to resources.icon() in create_treeitem() instead of
calling resources.icon() every time an icon is specified.

Signed-off-by: Daniel Harding <dharding@living180.net>
qtutils: rename some icon functions

Rename ident_file_type() to icon_name_for_filename() and icon_file() to
icon_name_for_file() to better reflect functionality.

Signed-off-by: Daniel Harding <dharding@living180.net>
qtutils: use smaller icon for consistency

daf997be70dd51547eb431702fa9900bccd1ad3b added a new smaller staged icon
for the status widget, but create_treeitem() was not updated to use the
new icon when called with check=False.  Update create_treeitem() to use
the smaller icon.

Signed-off-by: Daniel Harding <dharding@living180.net>
widgets/status: use icon for untracked files

git-cola has an icon for untracked files, but it wasn't being used,
because the StatusTreeWidget was not providing the correct argument to
qtutils.create_treeitem().  Change StatusTreeWidget.set_untracked() to
pass untracked=True to StatusTreeWidget._set_subtree(), which will pass
that value on to qtutils.create_treeitem(), causing the desired icon to
be used.

Signed-off-by: Daniel Harding <dharding@living180.net>
Merge pull request #388 from living180/icon_cleanups

Icon cleanups

Signed-off-by: David Aguilar <davvid@gmail.com>