We set the UI to change from 'downloading' state to 'processing' state,
and now we give the UI the processor time to actually show this new
state on the screen before we start that processing step.
The ImportWalletPage now loads the QML provided by the module,
exposed via the metadata of modules.
The module gets just a blockheight property and then will do
"its thing".
This will either instantly close when there is nothing to do
and continue instantly to the actual import.
Or the module will check the server, initiate download and when
all is setup and done THEN close the popup and continue with
the actual import.
This actually checks the need for downloading more headers,
then decides on which headers to download (from which known checkpoint,
really) and downloads them to a file.
Plugins can be plugged into the main menu and the send menu already,
this adds the ability for a (section of a) plugin to be loaded as
custom component in the main app, among others.