[config] extracting reusable part

Signed-off-by: Denis Zalevskiy <denis.zalevskiy@jolla.com>
[cmake] install exact public headers

Signed-off-by: Denis Zalevskiy <denis.zalevskiy@jolla.com>
[config] touch configuration directory

to be sure watcher is kicked

Signed-off-by: Denis Zalevskiy <denis.zalevskiy@jolla.com>
[config] using mtime to compare config files

Signed-off-by: Denis Zalevskiy <denis.zalevskiy@jolla.com>
[config] first removing provider, then adding

Signed-off-by: Denis Zalevskiy <denis.zalevskiy@jolla.com>
[server] check plugin pointer on add/rm

Signed-off-by: Denis Zalevskiy <denis.zalevskiy@jolla.com>