Now the status bar behavior is much more reliable on X11 systems, no more
lags when moving GoldenDict window around or when resizing it. Works
better with compositing window managers as well.
Also, the new status bar also supports images, which is good for
warnings and error messages.
The status bar is fully styled and can change its appearence
based on user's CSS, if needed.
* Small pop-up window at the bottom of the main winodw instead
of traditional status bar, that consumes lots of space.
* API, similar to standard Qt's status bar.
* The status bar hides itself after specified amount of time.
* Clicking on the status bar also hides it.
* Properly behavies on resizes/moves/focus/etc.
* Tested on Linux and Windows.
