Implement HalfPageView

[cecilios]

Its need was discussed in #233.

It will have a double behaviour:

• In normal mode (no playback) it will behave as SinglePageView: a single page as high as necessary to contain all the score.
• When entered in playback mode, the bitmap to be rendered in the application window is split horizontally in two halves, originating two virtual vertical windows, one at top and the other at bottom. In the top window it is displayed the first score chunk and the second chunk is displayed in the bottom window. When playback enters in the bottom window the top window is updated with the third chunk. Next, when the playback enters again in the top window, the bottom window is updated with the next chunk, and so on.
• When playback is finished (because the end of the score is reached or because the app stops playback  -- but not when paused --) the display returns automatically to SinglePageView.

This view allows to solve the problem of repetition marks and jumps: when the window being played has a jump, the other window is updated with the next logical system positioned at right measure instead of with the next chunk. The behaviour for the user is simple: when the user finds a jump he/she will know that it is necessary to jump to the other half window, unless the repetition is over and the music should continue from that point.