Allow custom "Next" button per page

Allow pages to set a custom text for the forward button, instead of
default "Next"

[endlessm/eos-shell#3594]
endless-eula: Remove terms checkbox

[endlessm/eos-shell#3594]
endless-eula: Set "Accept and Continue" as forward text button

[endlessm/eos-shell#3594]
Merge pull request #63 from endlessm/shell/3594

Change Endless EULA checkbox by button