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Homogenize API. All function in core/ext return int or unsigned int
instead of size_t  or other  irt::i32  etc. (Closes: #76).

- size() -> unsigned
- ssize() -> int
- capacity() -> int
- init(int)
- resize(int)
- can_alloc(int)

In menu `View`, `Fix window layout` disable, `Merge main editors`
disable, the Components list  and  Observation windows flick when
simulation or modeling editors are selected.

We fix the problem using tab bar and multiples windows according to the
`Fix window layout` and `Merge main editor` boolean values (Closes: #90).

Bad use of `ImGui::End()` for project, log, observation and components
windows (Closes: #89).