Query editor: Ctrl+Enter executes only selection
Query editor: Ctrl+Enter executes only selection

This CL makes the following change: if there is no selection, Ctrl+Enter
executes the whole query box as usual. Otherwise, only selection is
executed. This change makes the experimentation with different queries
significantly easier: instead of swapping different queries via an
external editor, several can reside in the editor at every moment and
executed as needed.

Change-Id: Id63e50d7cc3151c9c5952bb8e1cd917504d8a9c6
google/perfetto,google/perfetto,google/perfetto,google/perfetto,google/perfetto,google/perfetto,google/perfetto,google/perfetto