ImguiIntegration: support for imgui IO event API

[pezcode]

This adds support for the new IO handling about to land in imgui 1.87. The main advantages:

• Full range of keys that roughly matches the Application classes' KeyEvent::Key, meaning you can do key handling with imgui, independent of the Application class
• Input events are trickled, e.g. mouse button down followed by mouse button up in the same frame is now spaced out over two frames. This makes the manual workaround for mouse buttons obsolete and fixes the same issue for keys.

A few notes:

• There is full backward compatibility with older imgui versions, using the legacy IO interface
• These changes make https://github.com/mosra/magnum-integration/pull/89 obsolete
• [x] Depends on https://github.com/mosra/magnum/pull/547 so all Application classes support the entire range of ImGuiKey

Todo:

• [x] Test with 1.87 final and update the API version used to check for the new IO API

[codecov[bot]]

Codecov Report

Merging #93 (a29dea2) into master (da3c1ab) will decrease coverage by 0.19%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master      #93      +/-   ##
==========================================
- Coverage   77.25%   77.06%   -0.20%     
==========================================
  Files          21       21              
  Lines         941      933       -8     
==========================================
- Hits          727      719       -8     
  Misses        214      214              

Impacted Files	Coverage Δ
src/Magnum/ImGuiIntegration/Context.cpp	89.13% <100.00%> (-1.13%)	:arrow_down:
src/Magnum/ImGuiIntegration/Context.hpp	86.48% <100.00%> (+1.63%)	:arrow_up:

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[mosra]

Merged as c60149c818bbdf61d97f30706f2018d968930718 (I squashed all changes together since it didn't seem to make sense to preserve the later-removed temporary hack), thanks a lot!