[Qt] minor receive tab improvements

- draw border around upper form
- align clear button with form elements
- set stretch on the Requested payments table in vertical layout.
  This is to have better looking and more useful behavior, when resizing/maximizing the window
- show tooltips also for the labels itself (label,amount,message)
- show (no amount) instead of 0.00, similar to (no label) and (no message)
- add keyboard support for pressing return.
  When upper form focused and return pressed -> submit form (Request payment)
- rename "Requested payments" to "Requested payments history"

@laanwj
What do you think about removing the "Requested payments" table from the front page and put it in a popup instead, by clicking a button "Requested payments history".
Currently the table looks very important, but its only a history, in case you want to look something up.
Also when adding the real payment request to the form, we need more space anyway...

Before:
![receive1](https://f.cloud.github.com/assets/2814559/2003390/26574e76-864d-11e3-859b-1ee367b8b0d8.png)

After:
![receive2](https://f.cloud.github.com/assets/2814559/2003391/2cace2fe-864d-11e3-9bec-6e9b87ae1ccf.png)