wxgui: Fixes to solve issues using GL sinks on OS X

plotter/plotter_base.py:

* Create explicit OpenGL context, check if window is shown before
realising context & only create PaintDC is paint triggered by WM
message (OS paint event)

* Added SetSizeHints to WX sink windows so sizing occurs correctly
wxgui: fix issue with flickering on Linux created by bea6d506
cmake: make install directory of GNU Radio cmakes files configurable
blocks: fix big-endian issue in wavefile blocks
blocks: fix param test in keep_me_in_n GRC wrapper
core: ensure message passing blocks marked done are handled
Merge branch 'fix-wxgui-gl' into maint