Sanitize variable types

The many type casts were causing a real mess. The new approach seems
to have the same level of error proneness, while the compiler finally
reports warnings about wrong types.
Move the chart's web view to the UI definition

Fially! Thanks goes to tristan, KaL and ebassi on #gtk for the
guidance.
Move selection toolbar into tab_list

It is not used anywhere else, there is no need to include it in the main
wrapping grid.
Add chart loading progress bar