implement an interactive app for Raster Masking

[janzandr]

Discussed with @jakimowb and @thielfab.

Raster Masking includes 2 main steps: a) Build Mask and b) Apply Mask.

Different usecases for building a mask should be supported:

• thresholding, e.g. array > 1000, array <= 1000, 100 < array <= 1000
• selecting categories from a classification, like implemented in GEE TSE app:
• selecting categories from a bitflag band, like implemented in GEE TSE app:
• more usecases?

The mask building process should be interactive, so that after selecting a new threshold or a category, the updated mask is visualized inside a map view.

Optionally, instead of visualizing the mask, the user can select a raster to automatically apply the mask.

[aloboa]

Actually, Raster Data Plotting is very close to an ideal solution. Just

1. Add the option of saving the polygons that are drawn over the image whenever a polygon is drawn in the bivariate spectral plot.
2. For the univariate case (1 single band), display the histogram and let the user select segments on it.