Obey system colors for dark theme support

[HebaruSan]

Problem

You can configure the legacy system colors on Windows to change how WinForms apps display. In Windows 7:

1. Right click desktop
2. Personalize
3. Window Color (in the bottom middle)
4. Advanced appearance settings...
5. Choose the colors that you want to use

CKAN doesn't look very good in this mode:

Note that it looks OK if you choose one of the more extreme "high contrast" themes, but these also change your window borders and everything else, so it's not a great solution:

Causes

• A number of the WinForms controls default to a hard coded style instead of obeying the system theme
• The status bar needed its BackColor/ForeColor properties set

Changes

• The mod list has visual styles turned off and its header color set to obey the system colors
• ThemedListView and ThemedTabControl classes are created as drop-in replacements for ListView and TabControl but tweaked to obey system colors
• The status bar's BackColor and ForeColor are set

This makes dark themes on Windows look better:

It also results in a minor degradation of appearance in a normal theme because the gradients for inactive tabs and list headers are suppressed:

Needs to be tested on Linux. I think it should look the same, but there could be surprises.

Fixes #2881.

[DasSkelett]

On Linux, it is now really hard to recognize the active tab in the tab panels:

[HebaruSan]

I guess .NET considers the tab's "bounds" to be the space inside the border, while Mono considers it to include the border? Fix pushed, need to test in Windows. Hopefully we don't have to put Platform.IsMono checks in here to make it work on both.

[HebaruSan]

Worse than I hoped, better than I feared; there's a terrible looking 1px line under the tabs now in Windows:

... but otherwise it's serviceable. Hopefully translating the box down 1px will solve that.

[HebaruSan]

Yup, offsetting down by 1 worked.

[DasSkelett]

Looks okay again now.
Did not test on Windows, but your screenshots on W7 look good.
According to this article, W10 users have to do some registry editing to change the colours: https://winaero.com/blog/change-window-background-color-in-windows-10/
Maybe someone will try it out one day and report if it works too.