Add shortcut button rescale custom range

The rescale to custom range is a very commonly used feature, but it is burried
on the "Color Map Editor Panel". A new button for this feature has been added
in the "Active Variable Controls" toolbar, like the "Rescale to Data Range"
one.

Change-Id: Idd24b88f91ded709af7a3121482c61e8829dd68a
Make discretize color more prominent

The option to "Discretize" the colors with its associated "Number of Table
Values" is regularly used but hard to find. It is now more accessible
in a "Color Discretization" group.

Change-Id: Id05c99a90c887f8ce4f8269c17d2b0def885c779
Fixed checks for drawable when rendering.

Fixed check for drawable to work properly for context views as well (not
  just render views). Fixing logic added by commits c23602752 and
c8736fa3. vtkSMViewProxy should not have code that works with
vtkPVRenderView alone. This fixes LoadStateMultiView test issues on OsX.

Change-Id: I8ef899cb47fafae75fa57ef0a603c7a12e48e00e
Merge topic 'fix_osx_test_issues' into next

47198123 Fixed checks for drawable when rendering.
Fix Sphinx documentation.

The API changes page is not being linked to correctly. Fixed that
(hopefully).

Change-Id: I3454da48c9d41c7a30f5f745aa389ed1bcd50f31
Merge topic 'fix_osx_test_issues' into next

0c935965 Fix Sphinx documentation.
Merge topic 'rescale_custom_range' into next

cd60f685 Add shortcut button rescale custom range
Merge topic 'discretize_color' into next

0ce9fad4 Make discretize color more prominent