qt: improve "receive coins" workflow

This is a work in progress to improve the workflow for receive coins and fix #2429.

New receive tab:
![example1](https://f.cloud.github.com/assets/126646/1342888/e2ed7e8a-366a-11e3-9392-ddb8a39f0832.png)

After clicking "Request payment" a new receiving address is created (except when "Reuse ..." is checked, in which case the user can select an existing one). Then the following window is shown:

![newreq](https://f.cloud.github.com/assets/126646/1360439/c9ac093c-37f5-11e3-972e-81ddc0b1102d.png)

When compiling without QR codes, the dialog is the same apart from the QR code at the top.

I'm thinking of putting address/amount/label/message in a single text area to make it easier to copy/paste the entire payment request to the recipient (for example into a mail), but this is the general idea.

I've also simplified the interface by removing the "addresses" tab and instead moved command for getting a list of used receiving and sending addresses to menu 
    ![menu](https://f.cloud.github.com/assets/126646/1342923/88fbd0b0-366b-11e3-9575-b1b53f7a403b.png)

Any testing is welcome. Known currently broken:
- Sign/verify message from address list