qt: add Open URI dialog

This adds a simple "Open URI..." option to the menu which allows manually opening URIs and payment requests from files on disk.

![openurl1](https://f.cloud.github.com/assets/126646/1482933/a436f73c-46ed-11e3-9a17-c05f9d03c62c.png)
![openurl2](https://f.cloud.github.com/assets/126646/1482934/a448fe5a-46ed-11e3-8086-eda85889fec3.png)

Some behind-the-scenes code changes:

- Always route received URIs (even those drag/dropped) through PaymentServer; this means that bitcoin: URIs that refer to payment requests can be used
- Add getOpenFileName function to GUIUtil analogous to getSaveFileName

Addresses #3202.