UI for untrusted devices, encrypted folder sharing (fixes #724)

[Catfriend1]

Purpose:

• Support featureFlag "encryption" (fixes v1.12.x compat - Support untrusted devices and encrypted folders #724)

Known issues, maybe fullfilled by a separate, future PR:

• Folder edit dialog: missing a nicer way to present the password field

Status:

• Device edit dialog: Add the password entry part when shared folders are selected from the dialog.
• Folder Accept notification needs to open the CreateFolder dialog with "receiveEncrypted" type suggestion. We need to implement "pendingFoldersChanged" for this to work.
• Pending device and pending folder events from SyncthingNative v1.13+ are supported and fields "receiveEncrypted", "remoteEncrypted" are gathered from these events.
• Folder edit dialog: Show "if untrusted, enter password" text field
• folder overview UI: Show "delete unexpected changes" button - revert button for receive encrypted
• Disable MediaScanner routine after files from "receiveEncrypted" folders were received.
• Disallow switching existing folder's type to "Receive Encrypted"
• Add "receiveEncrypted" folder type to UI
• Added "ID" icon to clarify which fields the folder / device ID should be put in.
• Removed the "key" and the "tag" icon from the folder edit dialog, replaced by "A-Z" icon to clarify which is the device name and which will be later the folder encryption password.
• Updated model: "Folder" class now contains the relevant info about devices the folder can be shared with in a new class "SharedWithDevice" (instead of "Device").
• Updated model: "Device" class now knows about the "untrusted" property.
• Updated model: The "options" config section now know about the "featureFlag" property.

Screenshots:

[ginkel]

Works like a charm, thank you very much! :-)