add bindable text path to BindableButton component

[DenisZhukovski]

[DenisZhukovski]

@ChebanovDD could you please take a look

[DenisZhukovski]

Why not simply use BindableLabel?

<ui:UXML xmlns:uitk="UnityMvvmToolkit.UITK.BindableUIElements" ...>
    <uitk:BindableButton ...>
        <uitk:BindableLabel ... /> <!--Using BindableLabel to display button text.-->
    </uitk:BindableButton>
</ui:UXML>

As for me it gives better flexibility, Button already has text property and why not to bind on it? In the future button implementation may be changed and we will be independent. Also internal button label can have some style class names automatically assigned which you may forget once do it manually