Previously we hooked into the window's key-press-event signal. This was
fine when there was only one widget... :)

But it turns out that if we make the layout focusable, and give it the
window focus, then we can take advantage of GtkScrolledWindow's built-in
[Ctrl-]PageUp/PageDown/Home/End support (as well as
Ctrl-Up/Dn/Left/Right) and only need to do our own thing for bare arrows
plus spacebar. There is currently no indication that the diagram has
focus, but nor is there in my web browser. :)

This lets us fit way more on the screen. Plus, emboldening the method
names makes it easier to skimread.

These icons are GPL v2 or later. They're too monochrome for my tastes
anyway.

Things this does not include:

 • a way to show/hide the side pane. Frustratingly gtk2hs doesn't
   support adding action widgets to notebooks yet so i can't put a close
   button beside the tabs. but I guess we need a menu item anyway.
 • ellipsised text doesn't right-justify properly, which is probably
   Pango's fault.
 • rendering a method/signal icon
 • filter out messages to the bus daemon

Fixes the following failure:

    cc -O2 -pipe  -Wl,--hash-style=gnu -Wl,-O1 -Wl,--as-needed \
            -g -O2 `pkg-config --cflags --libs dbus-1` \
            -Wall -Wunused \
            -o bustle-dbus-monitor bustle-dbus-monitor.c
    bustle-0.2.3/temp/ccodU65H.o: In function `main':
    bustle-0.2.3/work/bustle-0.2.3/bustle-dbus-monitor.c:347: undefined reference to `dbus_error_init'
    bustle-0.2.3/work/bustle-0.2.3/bustle-dbus-monitor.c:348: undefined reference to `dbus_bus_get'
    bustle-0.2.3/temp/ccodU65H.o: In function `get_well_known_names':
    bustle-0.2.3/work/bustle-0.2.3/bustle-dbus-monitor.c:241: undefined reference to `dbus_message_new_method_call'
    bustle-0.2.3/work/bustle-0.2.3/bustle-dbus-monitor.c:249: undefined reference to `dbus_error_init'
    bustle-0.2.3/work/bustle-0.2.3/bustle-dbus-monitor.c:250: undefined reference to `dbus_connection_send_with_reply_and_block'
    bustle-0.2.3/work/bustle-0.2.3/bustle-dbus-monitor.c:252: undefined reference to `dbus_error_is_set'

Signed-off-by: Sergei Trofimovich <st@anti-virus.by>
Signed-off-by: Will Thompson <will.thompson@collabora.co.uk>

It would be better to put these in the UI, but …

It's no use to the user to fail entirely; we should just warn them.