BColorControl: Size text rect based on font size

* Also change kMinCellSize from a uint32 to a float so that it can be used
  with std::min() and std::max() instead of min_c() and max_c().
* Set the text controls sizes and margins based on the font size. Also rework
  _TextRectOffset() so that it will get the right spacing from by dividing the
  palette frame by 3.
* Replace bare numbers and refactor with calculation or magic constant.
* Create a private method _TextRectOffset() which calculates and
  returns the vertical text rect offset to use based on the font size.
* Replace 2.0 with new kBevelSpacing constant where appropriate.
* fPaletteFrame calculation in _LayoutView() was refactored but should
  not have changed.

headers/os/interface/ColorControl.h

@@ -103,6 +103,7 @@ private:
 			BRect				_PaletteSelectorFrame(uint8 colorIndex) const;
 			BRect				_RampFrame(uint8 rampIndex) const;
 			void				_SetCellSize(float size);
+			float				_TextRectOffset();
 
 	private:
 			BRect				fPaletteFrame;