Bug 576377 - Provide shortcuts/commands for incremental
multiselection/multiple carets in text editors

Expansion of the multi-selection commands to consider the first
selection range as an anchor to which then subsequent command calls
relate. Thus it's now also possible to create a multi-selection
incrementally "upwards" from the initial anchor selection, and
also revert downwards.

Change-Id: Ica3e444064df9373fee65bd5f4b4bcf2cb146750
Reviewed-on: https://git.eclipse.org/r/c/platform/eclipse.platform.text/+/191824
Tested-by: Mickael Istria <mistria@redhat.com>
Reviewed-by: Mickael Istria <mistria@redhat.com>
10 files changed