Bug1669: Close and Menu buttons should never disappear...

... As they did with Ctrl+Shift+F, with a certain minimimum number of wave
tracks that depends on screen resolution.  On my Macbook, that was 13 tracks.

This bug was a consequence of James' TCP layout changes making buttons a little
taller plus my changes to hide any top controls for which there is not
sufficient height.

The fix makes the height of tracks after Ctrl+Shift+F a bit more (44 pixels
not 40) than in 2.1.3.

This fix also replaces some more "magic numbers" with calculation from
tabled data about TCP layout.

src/TrackPanel.cpp

@@ -1313,6 +1313,18 @@ std::pair< int, int > CalcBottomItemY
 
 }
 
+unsigned TrackInfo::MinimumTrackHeight()
+{
+   unsigned height = 0;
+   if (!commonTrackTCPLines.empty())
+      height += commonTrackTCPLines.front().height;
+   if (!commonTrackTCPBottomLines.empty())
+      height += commonTrackTCPBottomLines.front().height;
+   // + 1 prevents the top item from disappearing for want of enough space,
+   // according to the rules in HideTopItem.
+   return height + kTopMargin + kBottomMargin + 1;
+}
+
 bool TrackInfo::HideTopItem( const wxRect &rect, const wxRect &subRect,
                  int allowance ) {
    auto limit = CalcBottomItemY