Fix handling of signs placed by other plugins

It appears to address numerous amount of issues, including: #812, #801, #887.

Proposed solution uses dummy sign which is placed before updating
diagnostic signs and unplaced afterwards, which eliminates any
flickering. Also, it not just unplace all, it unplaces only that marks
that are changed, so performance should not be an issue in case of many
diagnostic messages.

It's common solution that can be found in some vim plugins that manage
signatures.

Signed-off-by: Stanislav Seletskiy <s.seletskiy@gmail.com>
Minor refactoring & stylistic changes.
Merge branch 'seletskiy-master2'