Transport endpoint enumeration

[fredzo]

Hi Andrew,

This is the 3rd PR of a set of 3 designed to add transport endpoint enumeration as proposed bu @sugarthc on scopehal Discord channel (goes with ngscopeclient/xptools#38 and ngscopeclient/scopehal#1072).
This scopehal-apps part covers:

• Adding a new "Endpoint" Combo in AddInstrument dialog that is displayed if selected transport supports device discovery
• For UART endpoints, the first part of the path (com port) is automatically updated according to the selected endpoint (path can still be edited manually)
• For HID endpoints, the path is filled by the vendorId/productId/serialNumber corresponding to the selected device.
• A refresh button has been added beside the Endpoint combo to allow user updating the combo when a new device is connected