[FEAT] General improvements for default styles management

[tbouffard]

Is your feature request related to a problem? Please describe.
This is a top level issue to discuss ways we could introduce to easily manage several use cases related to BPMN styles and themes.

Currently in version 0.29.1, we let users customize the styles and theme with hacks:

• changing defaults (like colors) require to update bpmn-visualization internals (by changing the StyleDefault enum)
• redefining styles implies calling mxGraph native code and knowing when to call it (after lib initialization)
• changing the theme dynamically require extending the BpmnVisualization class and calling mxGraph native code (see the Hacktoberfest demo or the bpm-visualization "load diagram" demo)

Describe the solution you'd like

List of use cases

Note: a lot of use cases are mentioned in #953, in particular live style changes done for the November 2020 POC (#869)

• Setting the defaults
• Change the default style configuration
• Define various BPMN Themes
• Apply style configuration change or a BPMN Theme to the rendering
• Reset rendering to the default styles/theme

Possible implementation

Introduce a new "style" API

methods:

• configure((style: mxStyleSheet) => void): void
• reset(options): void
• setDefaults: at lib initialization (see [FEAT] Provide a way to configure the defaults of the BPMN renderer #1993). No real need at runtime, if required, this can be done using the "configure" methods (except of overlays defaults, but is there a real need for that?).
• refreshRendering: to apply style changes done with the "configure" method

configure:

• avoid class extension or calling mxGraph native code
• if we move to maxGraph (see refactor: [POC] Replace mxGraph with maxGraph 0.1.0 #2366), there will be some minor breaking changes for implementor
• calling this method doesn't change the rendering
• the passed function parameter can update all or some styles
• the method can be called several times

reset / resetStyleDefinitions / resetToDefaults:

• facilitate switch to another Theme
• reset use the defaults defined by the user

possible implementation (see also #2458)

• call styleSheet.styles and remove all entries. How do we do for defaultVertex and defaultEdge ?
• recreate all default styles with StyleConfigurator
• check if mxGraph provides an API for that

To check: what will happen to elements that received specific style changes with #1030 and #2468? Probably removed

Alternative: enrich some existing options

Changing the initial default may be done using the GlobalOptions or LoadOptions, see #1993

Existing issues that can benefit from the decision taken here

Beta Give feedback

1. [FEAT] Provide a way to configure the defaults of the BPMN renderer #1993
   BPMN diagram styling enhancement +2
2. [FEAT] Provide an option to ignore BPMN LabelStyles #2468
   BPMN diagram styling enhancement +2
3. [FEAT] Provide an option to ignore BPMN Label Bounds of activities #2469
   BPMN diagram styling enhancement +2
4. [REFACTOR] Keep the loaded BpmnModel into memory #953
   refactoring +1