Exclude layer from rendering

Implemented the possibility of exclusion layers from rendering. Excluded
layer will still visible in the editor.

Context class was changed. Now, it requires certain parameters of
rendering - ContextParams.

Class ContextParams for now contains only one field -
render_excluded_context. Which shows that this context intended for the
editor (show excluded layers) or for final rendering (hide excluded
layers).

ContextParams not needed for some operations with the context - set_time
() and set_dirty_outlines (). These functions placed into separate class -
IndependentContext. This class is the parent of the Context class.

Context and IndependentContext for canvas can be obtained by the methods
get_context () and get_independent_context ()

Function Canvas :: find_layer () now requires a context parameters.

The class hierarchy:

CanvasBase::const_iterator
     |
IndependentContext
     |
Context

You may toggle state of "exclude_from_rendering" flag by right click to the layer in layers tree (new item in menu). Labels of excluded layers written by italic font.