This adds a mini-framework for display of extra icons in layer tree views
next to layer and group names. Tool tip text can be associated with indicators
to give extra context for indicators. In addition, a signal gets emitted
when user clicks indicators and custom actions can be defined.
The main window's layer tree view (ToC) gets support for indicators
that are shown when a vector layer has a filter applied. This makes it easier
for users to understand that they are looking at a subset of all data.
Clicking the indicator's icon brings up query builder.
