FR-7: Handle program start with parameters passed (files to encrypt)

[skarpushin]

HTD:

• Open file explorer in Windows
• Right-click on any file
• There is a menu item in the appeared context menu: PGP Encrypt (or alternatively Send To -> PGP Encrypt)
• Click on this menu item
• Expected: PGP Vault app launched, Encrypt dialog opened, Input file appears in the "Source file" input field
• Expect: if this is first app start and there no keys imported yet then app will suggest to import keys first
• Expect (for MVP version only) if multiple files selected then Encrypt window will appear for the 1st file only (later we'll implement handling for multiple files at a time Feature: Allow to decrypt multiple files at a time #9 )