don't return the spending methods pay_invoice and multi_pay_invoice in
the get_info request and the info event so connections can be used for
services that enforce receive only connections.
store all plugin data by plugin name in a root dictionary `plugin_data`
inside the wallet db so that plugin data can get deleted again.
Prunes the data of plugins from the wallet db on wallet stop if the
plugin is not installed anymore.
- both internal and external plugins require GUI install
(except internal HW plugins, which are 'auto-loaded' and hidden)
- remove init_qt hook
- in Qt, reload wallet windows if plugin enabled/disabled
- add 'uninstall' button to PluginDialog
- add 'add plugins' button to wizard hw screen
- add icons to the plugin list
