[FIX] ContinuousPalettesModel: Disable 'category' items via flags

[ales-erjavec]

Issue

In Heatmap tab to the palette selection combo, change selection via up/down arrow keys.

-------------------------- AttributeError Exception ---------------------------
Traceback (most recent call last):
  File "/Users/aleserjavec/Documents/workspace/orange3/Orange/widgets/visualize/owheatmap.py", line 917, in update_color_schema
    self.palette_name = self.color_cb.currentData().name
AttributeError: 'NoneType' object has no attribute 'name'
-------------------------------------------------------------------------------

Description of changes

Disable 'category' items via model flags.

Includes

• Code changes
• Tests
• Documentation

[codecov]

Codecov Report

Merging #4538 into master will decrease coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #4538      +/-   ##
==========================================
- Coverage   83.37%   83.35%   -0.03%     
==========================================
  Files         279      274       -5     
  Lines       55755    54957     -798     
==========================================
- Hits        46485    45808     -677     
+ Misses       9270     9149     -121