Marker code cleanup

[jlitewski]

Cleaned up and unified the marker code I've been combing through.

• Renamed CursorXPos to g_MarkerXPos and moved all of them under ui.h/ui.c for consistency. It also is a better name for the variables due to them actually being markers and not cursors.
• Made the Marker drawing a bit more flexible by throwing it into its own function. It is now easier to add markers in the future. Note: I need to do that to the Annotation drawing code as well, but I'm not sure if the function should go in proxguiqt.cpp or ui.c or what. I honestly think it might be better to move the utility functions I've been creating into another file that's suited for that instead of gumming up the proxguiqt.cpp file more.
• Extended data setgraphmarkers to be able to modify Marker A and B as well as C and D. Also added --keep as a flag to prevent nuking other values not being set in a command if you don't want them to be.
• Renamed g_PlotGridXdefault/g_PlotGridYdefault to g_DefaultXGrid/g_DefaultYGrid. Frivolous change, but easier to read.

[github-actions[bot]]

You are welcome to add an entry to the CHANGELOG.md as well