Threaded feed updater (bug 153)

Threaded feed updater (bug 153)

This changeset is based on gPodder-r777-threaded-feed-updater.patch, plus:

config.py: Add new config option 'max_simulaneous_feeds_updating'. By default
it's set to 3, otherwise on slower devices the gui might become un-responsive
during an update because of the CPU resources required by python-feedparser.
Add 'color_updating_feeds' option for users that want to manually set it.

gui.py: Make channels that are updating a different color, this gives the user
a better idea of what's going on. Populate the update progress bar with a
message and clear the previous fraction setting before starting a feed update.
Colors dict for feed updates, default color is "None", so we don't need to do
any fancy color detection (for all different GTK themes).

libpodcasts.py: Make the channel's iter available as podcastChannel.iter.
Don't use the <span foreground=...> markup tag to set the channel's text
color, use the gtk.CellRendererText 'foreground' attribute. This allows
the text color to be easily changed by modifying the channel model.
Require the colors dict to be passed to channels_to_model.

For Maemo, add "Update selected podcast" menu item to the Subscriptions menu,
because the context menu isn't easily reached at the moment (no right-click).