UI hint of alternate function on long-tap

[denilsonsa]

The older Holo version of this calculator shows the alternate function at the top-right of each button, in a darker color.

Google Calculator has a INV button that switches sin/cos/tan/ln/log/√ to their inverse forms, and highlight the new functions in bold.

The current calculator has support for alternate functions for some of the buttons by long-tapping them, but there is no indication that such feature is available. At the first glance, a new user will assume the calculator is missing features such as asin.

[EmmanuelMess]

Working on it.

Edit: see #54