Added some new functionalities and changed the design

- The equals button is now blue (inspired by Googles Scientific
Calculator)
- Added “.” (dot) functionality. Still WIP since only one dot should be
present at each operand.
- Added two new ON and OFF buttons for enabling and disabling the
calculator easier and with less code.
- Added DEL functionality. The calculator now deletes the latest
character on the text view.
- WIP with the arrow keys. It should place the cursor to the left or
right.