Add splash screen to allow blockchain_headers to download

[ch4ot1c]

Currently, the app feels like it lags hard at the start - this is because this blockchain_headers file is downloading. We should introduce a splash screen to keep the UX pleasant.

[KaboomFox]

I have been following this issue in addition: spesmilo#3778

[GrannyCryptomaster]

Yep. Good thinking. The current bl..._h... take 374MB.

[tocorobo]

I don't know if others are experiencing this issue but the throughput to download the blockchain_headers file is extremely poor for me. If I open Resource Monitor and watch the outbound connections that the Electrum-ZCL-Z_1.0.3-win.exe initiates and I'm seeing like at most 24,000 to 30,000 Bytes/Sec. I see outbound connections to 40.115.22.134 and 167.244.154.104.bc.googleusercontent.com. Are those the only endpoints from which the blockchain_headers file is available right now? Are there other sources we can configure the client to pull from that might be faster?