More responsive line inspector

[corranwebster]

trafficstars

This modifies the line inspector to use the current mouse coordinates at draw time and update time, which helps with the appearance of responsiveness.

[coveralls]

Coverage increased (+0.01%) when pulling 7bf8fef37b73926e7b0954cc32602057779f677f on feature/responsive-line-inspector into 032f21a732e968616fc2bbb3016b0fffd7d51db2 on master.

[enbuilder]

Merged build triggered.

[coveralls]

Coverage decreased (-0.0%) when pulling d9ef23cfc3888aeb8a5b68f4aef8f0d51b1e245c on feature/responsive-line-inspector into 4faecdaf1d5de769ec57ef11b854e4a4ff5a7cae on master.

[enbuilder]

Merged build started.

[enbuilder]

Merged build finished.

[cfarrow]

It's a big pain, but I think this needs some tests. The drag tool tests give a decent starting point.