gui: Fix undefined behavior in TreeModel.
gui: Fix undefined behavior in TreeModel.

std::sort() requires the comparison function to return false for even
comparison. Returning true results in undefined behavior and a potential
segfault.

SymbiFlow/nextpnr,YosysHQ/nextpnr,SymbiFlow/nextpnr,YosysHQ/nextpnr,SymbiFlow/nextpnr,YosysHQ/nextpnr,SymbiFlow/nextpnr,YosysHQ/nextpnr