Problem:    Syntax timeout not used correctly.
Solution:   Do not pass the timeout to syntax_start() but set it explicitly.
            (Yasuhiro Matsumoto, closes #2139)

Problem:    Cannot define a toolbar for a window.
Solution:   Add a window-local toolbar.

Problem:    Terminal window does not use Normal colors.
Solution:   For the GUI and when 'termguicolors' is enabled, use the actual
            foreground and background colors for the terminal.  (Yasuhiro
            Matsumoto, closes #2067)
            Use the "Terminal" highlight group if defined.

Problem:    Terminal window cursor shape not supported in the GUI.
Solution:   Use the terminal window cursor shape in the GUI.

Problem:    With 8 colors the bold attribute is not set properly.
Solution:   Move setting HL_TABLE() out of lookup_color. (closes #1901)

Problem:    Terminal colors depend on the system.
Solution:   Use the highlight color lookup tables.

Problem:    Terminal window colors wrong with 'termguicolors'.
Solution:   Add 'termguicolors' support.

Problem:    Terminal window does not have colors in the GUI.
Solution:   Lookup the GUI color.

Problem:    Terminal window does not support colors.
Solution:   Lookup the color attribute.

Problem:    Syntax highlighting can make cause a freeze.
Solution:   Apply 'redrawtime' to syntax highlighting, per window.