Missing functionality for h5web plots

Now that h5web plots are the default i've come to realise some of the other bits of missing functionality:

For the MCAs (https://confluence.esrf.fr/display/UXUIT/Plotting+usage+and+capability#Plottingusageandcapability-mca):

• annotations (annotate ROIs on plot) !477 (merged)
• ability to zoom in and out (already present with the wheel, selection implemented in !473 (merged))
• Support for axis names !477 (merged)
• Requested by staff (low priority): draw region on plot and send coordinates in callback (to create a new ROI/annotation) !505 (merged)