qwt: update to 6.1.1
qwt: more files from updating to qwt 6.1.1
qwt: delete unused file
qwt: remove outdated constants

This is a pending change from qwt-6.1.2, and is needed to
support Qt 5.4
qwt: update qwt qmake files to build and link properly

fixes the target name, drops some unnecessary constants, etc.
scopes: adapt to renamed qwt 6.1 'set*Pen' methods
scopes: updates for qwt 6.1.1

There should be no change in functionality. How we interact
between the legend and plots had to change.