Nextpnr GUI support

[MartinGeisse]

Would it be possible to compile nextpnr, in particular nextpnr-ecp5, with GUI support?

According to https://github.com/YosysHQ/nextpnr GUI support is disabled by default to reduce dependencies. It is not immediately clear to me whether this is related to build-time dependencies or run-time dependencies. At run-time, the GUI must be enabled with a command-line option, but again it is not clear to me whether one can run nextpnr headless on a headless system just by ommitting that option once it was built with GUI support.

All this could imply that GUI support can only be provided by building two separate executables, nextpnr-ecp5 and nextpnr-ecp5-gui, for headless and gui mode, respectively.

With respect to ccompile-time dependencies, the manual says "On Ubuntu, install qt5-default", but this sounds like it would install the qt5 runtime on the build host, not the necessary compile-time files to build a qt5 executable that is intended to be run on a different machine. In other words, it seems to confuse the build host and the target machine.

[edbordin]

It's mainly left off at the moment because it was assumed that statically linking QT into two nextpnr binaries will add a significant amount to the overall package size. But I never actually checked how much so I'd be open to at least trying it and quantifying the size difference

[ArsenioDev]

Please do, I quite like the UI and would like it in these bins

[edbordin]

I decided months ago not to make any more changes to this package, I should have done a better job of communicating that, sorry! I've added a notice to the README.