TUI - general bindings - open transaction

Based on #261:
We should allow opening a transaction file (O for open) from any non-modal page. If we open a transaction and the cart has items, we should ask if user wants to abort or overwrite existing cart. If the transaction file loads successfully, open the transaction summary page.

To be consider:

1. Probably should be Ctrl + O instead of O (if it should be available everywhere, there is a problem with pages with editbox)
2. How the back/esc should work (problem with screen stack)