tdf#151190 sw content controls: make them read-write in protected sections

tdf#151190 sw content controls: make them read-write in protected sections

When the user picked a date in the date picker inside a protected
section, then we blocked the deletion of the placeholder, but allowed
insertion to the content control, which is inconsistent.

The trouble was that SwPaM::HasReadonlySel() detected that the cursor is
in a protected section, so deletion failed, leading to an ever growing
value of the content control.

Fix the problem by handling content controls similar to legacy form
fields: if the cursor is inside a content control, then that value is
editable, even if we're inside a protected section.

This is meant to fix all content control types, not just date pickers.

Change-Id: I9eca4c4f71dcfaa61d84414f7727adcd4496735c
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/141249
Reviewed-by: Miklos Vajna <vmiklos@collabora.com>
Tested-by: Jenkins