Unistroke bug in MessageView

[Josué C]

In the messageView of a class, drawing an unistroke opens a selectorView. When swapping scene without closing this selectorView, the unistroke is not displaying when opening the messageView again.

[Josué C]

References #523: fixed temporary components memory leak

→ <<cset 4332dab63aff>>

[Josué C]

Resolves #523: added a sub-class of SelectorView, allowing MessageViewHandler to revert it from temporary objects

→ <<cset 5cab4b1112a7>>

[Josué C]

References #523: Moved TemporarySelectorView class inside MessageViewHandler class

→ <<cset 91fdcd28854a>>

[Josué C]

References #523 : updated accessibility of class & methods

→ <<cset 0b8a4ba94228>>

[Josué C]

References #523: removed useless comments

→ <<cset cdfe795cfe45>>

[Josué C]

References #523: small changes for cleaning the class

→ <<cset 3b41359c114e>>