Refactored code responsible for high contrast mode. Now it allows to have more than one layer on the top.
Selecting layer using the dropdown list on the toolbar influences the layer displayed in high contrast mode.

Scripting engine build fix.

Eeschema: fix a *very subtle* bug noticeable only in complex hierachies, for multiple parts per package:
sometimes, the modified flag was set for these components just when switching from a sheet to an other sheet.
Pcbnew: fix Bug #1197414 (dragged track segments does not show clearance area)

Removed most of deprecated OpenGL calls. Items used to be drawn in immediate mode now are drawn using vertex arrays.

Added implementation of level of details per layer and item type pairs basis (items on every layer have a possibility to define the minimum VIEW scale to be shown).

Added the 'cached' parameter for VIEW_LAYER. The parameter decides if items drawn on the layer should be cached or drawn in immediate mode.
Removed m_useGroups from VIEW, as now groups are enabled per layer.

Fixed drawing of lines' caps.

Fixed drawing stroked semicircles using OpenGL backend.