inputbar left button: fixes text-only button not being displayed

[soscomp]

PR Summary

Fixes that left button is not displayed when using only the button label but no image.
When using only a label for the left button it previously wasn't displayed at all.
This was due to constraints recalculation was only done when the image is changed but not when the label is changed.
Now label is monitored for changes too and constraints are coreetly updated therefore displaying text-only buttons corretly.

Test strategy

Replace the image of the left button with some text. Previous version won't show the button this one will.