Fix tool strip border in dark theme

[NikolayXHD]

Fixes #8864

Screenshots

Before

After

Proposed changes

At the moment there is a couple of cases where a custom ToolStrip.Renderer solves a task not related to dark theme.

1. emulate context menu "caption" item, which serves as an item group header
   
   
2. remove ToolStrip borders (ToolStripEx class)

Current implementation of dark theme does not consider the cases above. It does

toolStrip.Renderer = new ThemeAwareToolStripRenderer();

to all toolstrips, which breaks the cases 1. 2. above.

This PR does such that if a ToolStrip needs custom rendering, it uses on of 2 predefined renderers,
ToolStripExRenderer or ToolStripExThemeAwareRenderer, both of which support scenarios 1. and 2. from above.

Why not use 1 renderer per 1 solved problem?

Because ToolStripRenderer design makes it impossible.

It offers 2 ways of customization:

• subscribe to an event such as RenderMenuItemBackground
• override a virtual method such as OnRenderMenuItemBackground

The event mechanism fails because it cannot suppress default painting, except the few cases when ther is an eventArgs.Handled field.

The inheritance mechanism does not allow you to compose different renderers so that each is responsible for solving 1 problem.

Amusingly, ToolStripRenderer does offer an ok way to compose renderers, which is a virtual property ToolStripRenderer.RendererOverride, but guess what, it's internal.

Every time I stumble upon virtual internal pattern in a library code, I cannot help feel that library developer considers other developers, especially me, second class.

Test environment(s)

• Windows10
• default dpi, 100%

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✒️ I contribute this code under The Developer Certificate of Origin.

[codecov]

Codecov Report

Merging #8899 (00dfa3f) into master (b4cbe02) will decrease coverage by 0.10%.
The diff coverage is 40.38%.

@@            Coverage Diff             @@
##           master    #8899      +/-   ##
==========================================
- Coverage   56.14%   56.04%   -0.11%     
==========================================
  Files         918      925       +7     
  Lines       65695    65899     +204     
  Branches    12055    12091      +36     
==========================================
+ Hits        36886    36934      +48     
- Misses      25795    25948     +153     
- Partials     3014     3017       +3     

Flag	Coverage Δ
production	43.27% <42.30%> (-0.09%)	⬇️
tests	94.83% <ø> (+0.01%)	⬆️

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[RussKie]

Awesome work!

I pushed few tweaks to API names, reordered methods, etc.

[ghost]

👍