Launch background Unit-e daemon in pruning mode

[cmihai]

Is your feature request related to a problem? Please describe
While the Unit-e blockchain will not be as large as the Bitcoin one at the beginning, a lower block time and higher TPS means it will grow much faster. The average desktop user can't afford to reserve hundreds of GB to store the whole blockchain history on disk, just to send and receive some UTE now and then.

Describe the solution you'd like
On the wallet setup page, the user should be prompted to enter the blockchain size on disk (>= 550 MB). This value should be saved in a configuration file, and the background Unit-e daemon should be launched in pruning mode with the specified target size.