meta.db: Entry points need adjustment

Entry points are currently named by the "canonical name" listed in the pPrettyName field. These names are intended for forward-compatibility cases. They are designed to be human-readable but only within the context of an entry point, having them "loose" like this is confusing.
Entry points are shown in the bottom-up and flat views. Even more confusingly they end up merged into a single row with 100% cost, with an arbitrary name in the scope column.

To Reproduce Occurs in all multithreaded meta.db databases.

Expected behavior Entry points should be named with names befitting their context:

When possible a proper name should be generated based on the entryPoint field (e.g. (entryPoint: 1) maps to <program root>).
If the entryPoint is unrecognized the pPrettyName should be housed in symbols to differentiate it from regular contexts (e.g. <entry: main thread>).

Entry points should not be present in any view besides the top-down view (and trace view).

Screenshots

Platform (please complete the following information):

HPCToolkit / hpcviewer.e4