Improve ax::Label class

[aismann]

This first entry will be updated if some news/changes/etc raising up.

[FIX]
SHRINK behavior:
#2503 Odd behavior with label overflow setting: Label::Overflow::SHRINK
[ ] shrink always to the dimensions (width)
[ ] No shrink if dimensions 0, 0
[ ] All text should be on ONE line
[ ] \n should always create a new line (shrink the text block now)
[ ] enableWrap(true) should create new lines on '\n' and 'Space' (shrink the text block now)

Add new tests: (at least: should not crashing):
[ ] dimension (all) is very small (e.g. 1,1) => what should happen?
[ ] dimension (height) is very small: (e.g. 0, 10 or 500, 10)

[Improve]
[ ] cleaner code
[ ] #2503 (comment) Label::updateContent() is called every single loop, even when the content has not changed