Fixing issues

[ImplOfAnImpl]

The following is fixed:

• Reject transactions with more than one reviewable input. This addresses the first part of Improve tx input review #14
• TxSummaryCollector::fees_iter will now fail with TxFeeUnderflow if an asset is only present in the outputs. This addresses the first part of Improve TxSummaryCollector #15
• Redundant images have been removed, together with the redundant image generation in the build script.
• The context and the UI state is now properly reset on errors etc, this addresses An error or an unrelated command should reset the current app context #12
• Message encoding stability tests were added, this addresses Need tests that ensure message encoding stability #16
• Change outputs are now recognized and marked as such, this addresses the second part of Change outputs should be omitted from review #17
• Update images, replacing Rust logo from the boilerplate app with Mintlayer logo - Replace all image files with ones containing the Mintlayer logo #10.