Some UI update patches

[sadrul]

I am going to attach a couple of patches in here for some minor UI updates:

• gtkui_preference_toggle_updates.patch:

Makes the filechooser buttons [in]sensitive depending on the state of the corresponding checkbuttons in the preference dialog. Does the same check for the 'Stop seeding' setting too.

• gtkui_disable_options_tab.patch:

When no torrent is selected in the torrentview, the tabs at the bottom (Statistics, Details etc.) are cleared, except 'Options'. This patch makes the child widgets within the tab insensitive. I also considered hiding the child widget, but making them insensitive looked better :)

• gtkui_update_main_window_torrents_menu.patch:

This updates the 'Torrent' menu in the main window as the selection in the torrent-view is changed. With the current patch, the menu is enabled only if at least one torrent is selected, otherwise it's disabled. The Pause/Resume/Remove menuitems should probably be disabled/enabled using the same logic as those used in toolbar.py:update_buttons. But this patch doesn't include that part, since I thought perhaps it'd be good to not have to duplicate the same logic in two places.

[andar]

Committed [3305]. Thanks!