Allow to set data breakpoints in memory view via context menu

[planger]

Description

Enable users to set data breakpoints for variables and memory addresses from the context menu of the following two places in the memory inspector:

To obtain the relevant context information, note that you may need to base this work on #135 (in particular to get the start address of a memory data group).

If a data breakpoint is set, it shall be shown in the UI with a red dot for variables, or a red rectangle with a dot for memory data groups. Make sure that this does not impact the existing width of the memory data groups to avoid line breaking or misaligned columns in the data. With #132 we already have accounted for a border (for editing), which can be also used now for showing breakpoints.

Optional: If a data breakpoint is hit, it should show up the memory range or variable in orange (similar to current line in a debug session). However, as it may be more complex to obtain this information (reason of the stopped event), we should be open to extract this out into a separate issue.

Additional information
Related: #51