[Qt] rescan progress

Fixes #3832.
If -rescan on startup, show progress in splashscreen, else show progress dialog.

Needed to implement a temporary notifications queue, otherwise the notifications
get queued and block the ui from updating the progress dialog.

Tested linux only qt4 and qt5.

![rescan2](https://f.cloud.github.com/assets/2814559/2484523/96c77e74-b10e-11e3-85b2-c789336036b6.png)

![rescan](https://f.cloud.github.com/assets/2814559/2484693/2995f810-b110-11e3-8352-78413351bec1.png)