UsesWidget: Always properly escape code for HTML

Some parts of the (non-highlighted) code didn't show up / were distorted in case they
contained '<' and/or '>'
Remove google sparse/dense hash code.

Replace the single place where it was used with a simple QSet.
Also cleanup the code, as this is not a hot-path thus the performance
is really not that important here.

Comparing before/after showed that DUContext::imports is roughly being
called 10k times when importing GammaRay as a project with a clean
cache. Parsing takes ca. 40s, of which 0.08s where spent in ::imports
with the old code. The new simpler QSet based variant takes ca. 0.1s
which is just as fine - esp. if we can get rid of so much code!

This cherry-pick was requested by FreeBSD packager rakuco.

(cherry picked from commit 64fe266b53a4655621609bd5bafeabbf76d10287)

Conflicts:
	language/duchain/ducontext.cpp
	language/duchain/ducontextdynamicdata.h
Minor: remove dead code.

Conflicts:
	language/duchain/duchain.cpp