Oculars plugin: Introduce a 5-value pluginMode property

[gzotti]

Description

The mode switching and zooming code in the Oculars plugin is hard to follow, with a series of intermingled side effects.
This work here should consolidate the state switching. However, it seems the menu buttons in the top-right GUI panel still need boolean properties. I have moved those as "slave properties" with private getters/setters.

• better understandable mode switching with one centralized method.
• no more entangled side effects
• cleanup not finished yet. For now I have commented away the previous functions for easier comparison.
• Any missing switching, storage and re-setting of main program settings should be done in the setPluginMode().

The other, even larger restructuring is the introduction of a dedicated Finder mode which replaces the hybrid isBinocular() from Ocular mode. This adds a button to the menu bar. However, Binoculars and Finders are collected in the same class "Finder" given that their properties are specified in the same way.

Fixes #1509, #356

Screenshots (if appropriate):

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• This change requires a documentation update

How Has This Been Tested?

Test Configuration:

• Operating system: Windows 10
• Graphics Card: irrelevant

Checklist:

• My code follows the code style of this project.
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules

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