Minor fixes for DQM classes.

A few months ago I did a simple test to test whether
having a huge library for all the DQM packages (instead of
.so per package) yield any performance results.

It did not.

However, I had to do some cleanup of the code in order to link it
correctly (mostly duplicate symbols):

1. L1TMenuHelper*.cc got de-duplicated;
2. add a proper headers, instead of including .cc (yeah, i know, crazy);
3. adding a few "static" keywords.
Add const to a few arrays.

Because we can!
Replace std::map::at with check if an element exists + at.

Previously it was done via operator[] (yes, indeed),
but this creates an entry and is no longer
possible (map is immutable).
Merge pull request #5716 from dmitrijus/biglibfixes2

Minor fixes for DQM classes.