Add Visual Style manager

[tig]

With

• the potential of a new Button look (Add 3D Shadow Effect for Button and other views #2144 / Fixes 2144. Add half-height bottom shadow for Effect3D. #2166) that may cause apps to break when enabled
• a proliferation of visual style settings on other Views (e.g. TreeStyle)
• the existing ColorScheme scheme and requests like User configurable Color Themes #29
• and more to come...

I suggest we invest in a Visual Style manager.

Basic idea:

• Build a JSON schema for the existing visual style settings on the various Views; it's important that this supports the library as it is NOW.
• Enhance existing View classes to have static "default" style properties (similar to what I did in my PR to Fixes 2144. Add half-height bottom shadow for Effect3D. #2166 that enabled UI Catalog to set the Button style globally.
• Build a component that reads a JSON doc and applies the specified styles such that all views that support it get their defaults set when the library loads.
• Build a mechanism for apps to configure per-app state
  • A visualstyle.json file that gets built into the app's assembly.
• Build a mechanism for storing user-defined per-app state.
  • A ~/.tui folder for global settings. ~/.tui/UICatalogApp.visualstyle.json would set the visual style of any app with a main class name of UICatalogApp. Unless...
  • A ./.tui folder for local settings.
    • ./.tui/UICatalogApp.visualstyle.json would set the visual style of any app with a main class name of UICatalogApp launched from anywhere under ./.
    • ./.tui/visualstyle.json would set the default visual style of any app launched from anywhere under ./.

I'm sure this has been invented before and we can plagiarize.

Let the debate begin! :-)

[tig]

Assume an app runs. The settings will be applied using the following precedence:

Priority descends in this order:

1. Global settings in home directory (~/.tui/visualstyle.json) -- highest priority
2. App settings in home directory (~/.tui/appname/visualstyle.json)
3. Global settings in local directories (./.tui/visualstyle.json)
4. App settings in local directories (./.tui/appname/visualstyle.json)
5. Settings in Terminal.Gui assembly -- lowest priority

The style manager first uses the settings from any copy of the visualstyle.json in the .tui/ directory in the user's home directory. Then it looks for any copies of the file located in the app directories, adding any settings found in them, but ignoring attributes already discovered in higher-priority locations. As a last resort, for any settings not explicitly assigned at either the global or app level, it assigns default values from the settings compiled into Terminal.Gui.dll.

(Up for debate/discussion)

[tig]

This is fixed by #2181