Flint: API to retrieve channel names from plot axis

Closes #3406 (closed)

This PR provides Flint API to the BLISS shell, in order to reach the actual channel used as axes, for the live curve plot and the live scatter plot.

Example

DEMO_SESSION [1]: flint()
DEMO_SESSION [2]: amesh(sy,-.75,.75,30,sz,-.75,.75,30,.001,fluo_diode)
DEMO_SESSION [3]: umv(slit_vertical_gap,.1);ascan(slit_vertical_offset,-1,1,30,.1,beamviewer)

DEMO_SESSION [4]: p = flint().get_live_plot("default-curve")
Current Flint PID:  13548

DEMO_SESSION [5]: p.xaxis_channel_name
         Out [5]: 'axis:slit_vertical_offset'

DEMO_SESSION [6]: p.xaxis_channel_name
         Out [6]: 'timer:elapsed_time'

DEMO_SESSION [7]: p = flint().get_live_plot("default-scatter")
Current Flint PID:  13548

DEMO_SESSION [8]: p.xaxis_channel_name
         Out [8]: 'axis:sy'

DEMO_SESSION [9]: p.yaxis_channel_name
         Out [9]: 'axis:sz'