It used to be that the control got stuck when adding and removing tags
in the tag input box which is now fixed. This is not a _minor_ thing,
although it sounds like it. From now on, the user should not be
confronted with unexpected behaviour.

As much as possible of the code uses MSnbase objects to keep data. For
example, instead of CSV files, the chromatograms are now inside a
Chromatograms object.

Things have been adapted to this until the preproc. The preproc stage
is still not working properly for MS2 part.

Logic in separating unknowns and knows was faulty.

R/mix.R (vald_comp_tab): New function. It checks for the existence of
ID, Name, RT(rt), SMILES and mz as needed in compound lists. It also
checks if IDs are unique, and if when SMILES, or mz are given, they
are not NAs.

Clarified the rtDelta description in QA box to mean RT shift tolerance
and added rtDeltaWin widget to spectra extraction box.

Retention time functionality in chromatograms.

Some naming changes.

R/extraction.R (gen_rt_range): Generates rt range matrix given rt-s
and tolerance.

This is where constants go.