Use single variable for location of tex file

Before this change we called `run-viewer` using a separate variable for
the directory and the the name of the tex file. We do not use the
directory name without the filename anywhere in the function. This
means we can use a simple interface for the function where we specify
the location of the tex file using only one variable.
Improve “Jump to Current Line in Viewer”

Before this change running “Jump to Current Line in Viewer” would jump
to the wrong position if we did not invoke the command on the master
file.