Enable nextpnr GUI support

[swittlich]

Description

Currently, the nextpnr flow runs in headless mode only.
It would be useful to enable the graphical user interface (GUI) of nextpnr for visualization and debugging purposes.

Expected Behavior

• Running nextpnr with GUI option (--gui) should open the graphical interface.
• The GUI should display placement, routing, and device resources.

Current Behavior

• Only command-line execution is available.
• No GUI window is shown when running nextpnr within the current setup.

Rationale

The nextpnr GUI is an important tool for:

• Visualizing placement and routing results.
• Debugging issues with constraints and resource usage.
• Providing an educational and user-friendly way to understand FPGA implementation.

Steps to Reproduce

1. Run the flow with nextpnr.
2. Try enabling the GUI (e.g., nextpnr-gatemate --gui).
3. Observe that no GUI window appears.

Environment

• Tool: nextpnr-gatemate
• Mode: headless (default)
• Desired: GUI mode support (--gui)

[swittlich]

Just a note for Windows users:

When running the nextpnr-* executables with GUI support, it is necessary to use the standard OSS CAD Suite installer (.exe).
The .zip package provided by OneWare should not be used for this purpose, since it does not include or set up the required Qt runtime environment.

If the .zip is used, the application fails to find the Qt platform plugin (qwindows.dll) and the GUI will not start, showing an error like:

qt.qpa.plugin: Could not find the Qt platform plugin "windows" in ""
This application failed to start because no Qt platform plugin could be initialized.

Using the official installer ensures that the correct Qt runtime environment is available and the GUI version of nextpnr works properly.
However, the official installer is currently not supported by OneWare, so this is the point where integration needs to be improved.