script: Add SetRefreshRate function

The script plugin hardcodes an FPS value of 50, which for some themes
and various devices is a bit much.  Add a new function
(Plymouth.SetRefreshRate) which sets the rate at which the
RefreshFunction gets called so each script theme can determine their most
appropriate rate.

https://bugs.freedesktop.org/show_bug.cgi?id=86247
utils: Don't create unix sockets non blocking

All the ply_read* functions assume the socket is doing blocking reads,
so opening unix sockets in non-blocking mode doesn't seem the best idea.

Specifically, this was causing ask-password to fail to read the response
at times as it got a -EAGAIN back from read rather then data.
main: Don't redirect output to /dev/null if we're tracing

Otherwise, our input will mysteriously disappear.
Remove the old log viewer tool
x11: Port to GTK+ 3.0

Port the remaining GTK+ code, the X11 renderer, to GTK+ 3.0.
throbber: don't report successful load when no assets

Currently, the throbber will report a successful load when zero assets
are loaded. Change it so that zero assets do not result in a
successfully loaded throbber.
throbber: don't report success when no frames were added

Currently, the throbber will report success when assets are *found* in
the directory. When loading those assets, code will apply more checks to
determine if the assets are meant for the throbber.
In case none of them are, we'll end up not adding any frames, so we
should not report a successful load.
two-step: Allow promptless / messageless boots

We don't have enough time to implement entries in our boot, and our
customers won't want them, so just hack the two-step plugin so that
we don't need them to load a theme.